What was the reason behind the fall of Buddhism in India?

Actually, there's a lot of factors regarding this. The Monks started to liberate the buddha's teaching. The Buddhism started to decline after the 4th buddhist council of India. Unlike King Asoka, King Kanishka of Kushan Empire, not keeping the precepts and still going on wars with other empires. He chose the most Sangha in the community back then the Sarvastivadin-Mahayana.

The last was because of the invasion of Turkic-muslim-Mamluk Dynasty which ended the Most Enormous Buddhist University, Nalanda.

As what Buddha said in DN16 Mahaparinibbana Sutta, there are 7 things x 7, which prevent decline in the community.

Now at that time venerable Ānanda was stood behind the Gracious One fanning the Gracious One. Then the Gracious One addressed venerable Ānanda, saying:

1) “Have you heard, Ānanda: ‘The Vajjians assemble regularly and assemble frequently?’” “I have heard this, reverend Sir: ‘The Vajjians assemble regularly and assemble frequently.’” “For as long, Ānanda, as the Vajjians will assemble regularly and assemble frequently surely growth, Ānanda, is to be expected for the Vajjians not decline.

2) Have you heard, Ānanda: ‘The Vajjians assemble unanimously, rise unanimously, and carry out their Vajjian duties unanimously?’” “I have heard this, reverend Sir, that the Vajjians assemble unanimously, rise unanimously, and carry out their Vajjian duties unanimously.” “For as long, Ānanda, as the Vajjians will assemble unanimously, rise unanimously, and carry out their Vajjian duties unanimously, surely growth, Ānanda, is to be expected for the Vajjians not decline.

3) Have you heard, Ānanda: ‘The Vajjians do not establish new laws that were not established, or cut off old laws that were established, and carry on with such laws as were accepted in the Ancient Vajjian Constitution?’” “I have heard this, reverend Sir: ‘The Vajjians do not establish new laws that were not established, or cut off old laws that were established, and they carry on with such laws as were accepted in the Ancient Vajjian Constitution.’” “For as long, Ānanda, as the Vajjians do not establish new laws that were not established, or cut off old laws that were established, and they carry on with such laws as were accepted in the Ancient Vajjian Constitution surely growth, Ānanda, is to be expected for the Vajjians not decline.

4) Have you heard, Ānanda: ‘The Vajjians honour the elders of the Vajjians, respect, revere, worship and think them worth listening to?’” “I have heard this, reverend Sir: ‘The Vajjians honour the elders of the Vajjians, respect, revere, worship and think them worth listening to.’” “For as long, Ānanda, as the Vajjians will honour the elders of the Vajjians, respect, revere, worship and think them worth listening to, surely growth, Ānanda, is to be expected for the Vajjians not decline.

5) Have you heard, Ānanda: ‘The Vajjians do not coerce and force their women and girls to dwell with them against their will?’” “I have heard this, reverend Sir: ‘The Vajjians do not coerce and force their women and girls to dwell with them against their will.’” “For as long, Ānanda, as the Vajjians will not coerce and force their women and girls to dwell with them against their will, surely growth, Ānanda, is to be expected for the Vajjians not decline.

6) Have you heard, Ānanda: ‘The Vajjians honour the Vajjian shrines amongst the Vajjians, both within and without the city, respect, revere, and worship them, and do not allow the righteous sacrifices that were formerly given, formerly made, to be neglected?’” “I have heard this, reverend Sir: ‘The Vajjians honour the Vajjian shrines amongst the Vajjians, both within and without the city, respect, revere, and worship them, and do not allow the righteous sacrifices that were formerly given, formerly made, to be neglected.” “For as long, Ānanda, as the Vajjians will honour the Vajjian shrines amongst the Vajjians, both within and without the city, respect, revere, and worship them, and do not allow the righteous sacrifices that were formerly given, formerly made, to be neglected surely growth, Ānanda, is to be expected for the Vajjians not decline.

7) Have you heard, Ānanda: ‘The Vajjians have made good arrangements in regard to the lawful protection, safety, and guarding of the Worthy Ones, so that Worthy Ones in the future can enter the realm, and having entered the Worthy Ones can live comfortably in the realm?” “I have heard this, reverend Sir: ‘The Vajjians have made good arrangements in regard to the lawful protection, safety, and guarding of the Worthy Ones, and that the Worthy Ones in the future can enter the realm, and having entered the Worthy Ones can live comfortably in the realm.” “For as long, Ānanda, as the Vajjians will make good arrangements in regard to the lawful protection, safety, and guarding of the Worthy Ones, and that the Worthy Ones in the future can enter the realm, and having entered, the Worthy Ones can live comfortably in the realm, surely growth, Ānanda, is to be expected for the Vajjians not decline.”

Then the Gracious One addressed the Magadhan chief minister the brahmin Vassakāra, saying: “At one time, brahmin, I was living near Vesālī near to the Sārandada Shrine and there I taught the Vajjians these seven things which prevent decline. For as long, brahmin, as the Vajjians maintain these seven things which prevent decline, and the Vajjians agree with these seven things which prevent decline, surely growth, brahmin, is to be expected for the Vajjians not decline.”

After this was said, the Magadhan chief minister the brahmin Vassakāra addressed the Gracious One, saying: “If the Vajjians, dear Gotama, were endowed with even one or the other of these seven things which prevent decline, surely growth is to be expected not decline, what to say about having seven things which prevent decline? The Vajjians cannot be overcome, dear Gotama, by the Magadhan King Ajātasattu, the son of Lady Wisdom, by war, but only through diplomacy, or through the breaking of an alliance. And now, dear Gotama, we shall go, as we have many duties, and there is much which ought to be done.”

“Now is the time, brahmin, for whatever you are thinking.” Then the Magadhan chief minister the brahmin Vassakāra, after greatly rejoicing and gladly receiving this word of the Gracious One, rose from his seat and departed.

3: Seven Things which Prevent Decline in the Community (1–7)

Then the Gracious One, not long after the Magadhan chief minister, the brahmin Vassakāra, had gone, addressed venerable Ānanda, saying: “Go, Ānanda, and whatever monks there are living in dependence on Rājagaha, assemble them in the Attendance Hall.”

“Very well, reverend Sir,” said venerable Ānanda, and after replying to the Gracious One, and assembling whatever monks there were living in dependence on Rājagaha in the Attendance Hall, he approached the Gracious One, and after approaching and worshipping the Gracious One, he stood on one side.

While standing on one side venerable Ānanda said this to the Gracious One: “The Community of monks have assembled, reverend Sir, now is the time, Gracious One, for whatever you are thinking.”

Then the Gracious One after rising from his seat approached the Attendance Hall, and after approaching he sat down on the prepared seat. While sitting the Gracious One addressed the monks, saying:

“I will teach you seven things which prevent decline, listen to it, apply your minds well, and I will speak.” “Very well, reverend Sir,” those monks replied to the Gracious One, and the Gracious One said this:

1) “For as long, monks, as the monks will assemble regularly and assemble frequently, surely growth, monks, is to be expected for the monks, not decline.

2) For as long, monks, as the monks will assemble unanimously, rise unanimously, and carry out their Community duties unanimously, surely growth, monks, is to be expected for the monks, not decline.

3) For as long, monks, as the monks do not establish new laws that were not established, or cut off old laws that were established, and they carry on with such training-rules as have been accepted, surely growth, monks, is to be expected for the monks, not decline.

4) For as long, monks, as the monks will honour the elder monks, those of long-standing, a long time gone-forth, the Fathers of the Community, the Leaders of the Community, respect, revere, worship and think them worth listening to, surely growth, monks, is to be expected for the monks, not decline.

5) For as long, monks, as the monks will not come under the influence of craving which has arisen for continued existence, surely growth, monks, is to be expected for the monks, not decline.

6) For as long, monks, as the monks will have desire for forest dwellings, surely growth, monks, is to be expected for the monks, not decline.

7) For as long, monks, as the monks individually will attend to the ways of mindfulness, so that their fellow celibates, who are well-behaved, in the future can come, and having come to their fellow celibates, who are well-behaved, can live comfortably, surely growth, monks, is to be expected for the monks, not decline.

For as long, monks, as the monks will maintain these seven things which prevent decline, and the monks will agree with these seven things which prevent decline, surely growth, monks, is to be expected for the monks, not decline.

4: Seven Further Things which Prevent Decline in the Community (8–14)

I will teach you a further seven things which prevent decline, listen to it, apply your minds well, and I will speak.”

“Very well, reverend Sir,” those monks replied to the Gracious One, and the Gracious One said this:

1) “For as long, monks, as the monks are not devoted to the pleasure in work, the delight in work, and are not attached to work, surely growth, monks, is to be expected for the monks, not decline.

2) For as long, monks, as the monks are not devoted to the pleasure in speech, the delight in speech, and are not attached to speech, surely growth, monks, is to be expected for the monks, not decline.

3) For as long, monks, as the monks are not devoted to the pleasure in sleep, the delight in sleep, and are not attached to sleep, surely growth, monks, is to be expected for the monks, not decline.

4) For as long, monks, as the monks are not devoted to the pleasure of company, the delight in company, and are not attached to company, surely growth, monks, is to be expected for the monks, not decline.

5) For as long, monks, as the monks do not have evil wishes, do not go under the influence of evil wishes, surely growth, monks, is to be expected for the monks, not decline.

6) For as long, monks, as the monks do not have wicked friends, do not have wicked companions, do not have wicked comrades, surely growth, monks, is to be expected for the monks, not decline.

7) For as long, monks, as the monks do not achieve only mundane or incomplete attainment, surely growth, monks, is to be expected for the monks, not decline.

For as long, monks, as the monks will maintain these seven things which prevent decline, and the monks will agree with these seven things which prevent decline, surely growth, monks, is to be expected for the monks, not decline.

5: Seven Further Things which Prevent Decline in the Community (15–21)

I will teach you a further seven things which prevent decline, listen to it, apply your minds well, and I will speak.”

“Very well, reverend Sir,” those monks replied to the Gracious One, and the Gracious One said this:

1) “For as long, monks, as the monks will have faith, surely growth, monks, is to be expected for the monks, not decline.

2) For as long, monks, as the monks will have a conscientious mind, surely growth, monks, is to be expected for the monks, not decline.

3) For as long, monks, as the monks will have a sense of shame, surely growth, monks, is to be expected for the monks, not decline.

4) For as long, monks, as the monks will be learned, surely growth, monks, is to be expected for the monks, not decline.

5) For as long, monks, as the monks will be strenuous, surely growth, monks, is to be expected for the monks, not decline.

6) For as long, monks, as the monks will attend to mindfulness, surely growth, monks, is to be expected for the monks, not decline.

7) For as long, monks, as the monks will possess wisdom, surely growth, monks, is to be expected for the monks, not decline.

For as long, monks, as the monks will maintain these seven things which prevent decline, and the monks will agree with these seven things which prevent decline, surely growth, monks, is to be expected for the monks, not decline.

6: Seven Further Things which Prevent Decline in the Community (22–28)

I will teach you a further seven things which prevent decline, listen to it, apply your minds well, and I will speak.”

“Very well, reverend Sir,” those monks replied to the Gracious One, and the Gracious One said this:

1) “For as long, monks, as the monks will develop the factor of Perfect Awakening that is Mindfulness, surely growth, monks, is to be expected for the monks, not decline.

2) For as long, monks, as the monks will develop the factor of Perfect Awakening that is Investigation of the nature of things, surely growth, monks, is to be expected for the monks, not decline.

3) For as long, monks, as the monks will develop the factor of Perfect Awakening that is Energy, surely growth, monks, is to be expected for the monks, not decline.

4) For as long, monks, as the monks will develop the factor of Perfect Awakening that is Rapture, surely growth, monks, is to be expected for the monks, not decline.

5) For as long, monks, as the monks will develop the factor of Perfect Awakening that is Calm, surely growth, monks, is to be expected for the monks, not decline.

6) For as long, monks, as the monks will develop the factor of Perfect Awakening that is Concentration, surely growth, monks, is to be expected for the monks, not decline.

7) For as long, monks, as the monks will develop the factor of Perfect Awakening that is Equanimity, surely growth, monks, is to be expected for the monks, not decline.

For as long, monks, as the monks will maintain these seven things which prevent decline, and the monks will agree with these seven things which prevent decline, surely growth, monks, is to be expected for the monks, not decline.

7: Seven Further Things which Prevent Decline in the Community (29–35)

I will teach you a further seven things which prevent decline, listen to it, apply your minds well, and I will speak.”

“Very well, reverend Sir,” those monks replied to the Gracious One, and the Gracious One said this:

1) “For as long, monks, as the monks will develop the perception of impermanence, surely growth, monks, is to be expected for the monks, not decline.

2) For as long, monks, as the monks will develop the perception of non-self, surely growth, monks, is to be expected for the monks, not decline.

3) For as long, monks, as the monks will develop the perception of the unattractive, surely growth, monks, is to be expected for the monks, not decline.

4) For as long, monks, as the monks will develop the perception of danger, surely growth, monks, is to be expected for the monks, not decline.

5) For as long, monks, as the monks will develop the perception of giving up, surely growth, monks, is to be expected for the monks, not decline.

6) For as long, monks, as the monks will develop the perception of dispassion, surely growth, monks, is to be expected for the monks, not decline.

7) For as long, monks, as the monks will develop the perception of cessation, surely growth, monks, is to be expected for the monks, not decline.

For as long, monks, as the monks will maintain these seven things which prevent decline, and the monks will agree with these seven things which prevent decline, surely growth, monks, is to be expected for the monks, not decline.

8: Six Things which Prevent Decline in the Community (36–41)

I will teach you a further six things which prevent decline, listen to it, apply your minds well, and I will speak.”

“Very well, reverend Sir,” those monks replied to the Gracious One, and the Gracious One said this:

1) “For as long, monks, as the monks with friendly actions by way of the body will serve their fellow celibates, both in public and in private, surely growth, monks, is to be expected for the monks, not decline.

2) For as long, monks, as the monks with friendly actions by way of speech will serve their fellow celibates, both in public and in private, surely growth, monks, is to be expected for the monks, not decline.

3) For as long, monks, as the monks with friendly actions by way of the mind will serve their fellow celibates, both in public and in private, surely growth, monks, is to be expected for the monks, not decline.

4) For as long, monks, as the monks in regard to those righteous gains, received in accordance with the Teaching—whatever amount has been received in the bowl—will divide and share such gains with those who are virtuous, fellow celibates, and share them in common, surely growth, monks, is to be expected for the monks, not decline.

5) For as long, monks, as the monks are endowed with those virtues which are unbroken, faultless, unspotted, unblemished, productive of freedom, praised by the wise, not clung to, leading to concentration, and will live endowed with virtue amongst their fellow celibates who themselves possess such virtue, both in public and in private, surely growth, monks, is to be expected for the monks, not decline.

6) For as long, monks, as the monks are endowed with that which is Ariyan View, which leads out, which leads to the complete destruction of suffering for one who acts thus, and will live endowed with Right View amongst those who themselves possess such Right View, both in public and in private, surely growth, monks, is to be expected for the monks, not decline.

For as long, monks, as the monks will maintain these six things which prevent decline, and the monks will agree with these six things which prevent decline, surely growth, monks, is to be expected for the monks, not decline.”

There also the Gracious One, while living near Rājagaha on the Vultures’ Peak Mountain, spoke frequently to the monks about the Teaching, saying:

“Such is virtue, such is concentration, such is wisdom, when virtue is well-developed it yields great fruit and brings great advantages in regard to concentration, when concentration is well-developed it yields great fruit and brings great advantages in regard to wisdom, when wisdom is well-developed the mind is completely liberated from the pollutants, that is to say: the pollutant of sensuality, the pollutant of craving for continued existence, the pollutant of ignorance.”

The declination of buddhism was because of breaking these.

Hopefully this helps. If you want to know more about this, you can try to read this book called "The concise history of buddhism".

After the Pari-nibbhana of The Buddha, there was a decline of Theravada Buddhism ( those who follow the teaching preserved in the Pāli), and a rise of the Mahayana Buddhism. The third council got underway to make the monastic order of those who follow the teaching preserved in the Pāli. With time Mahayana Buddhism rose up again. It was a death blow to those who wanted to keep the teaching preserved in the Pāli. Mahayanists translated the Dhamma into Sanskrit and rejected attaining of the Marga- phala of the Theravada School.

At the time only the Brahmins’ Veda was written in Veda and Sanskrit languages. Brahmins never felt that other religions will be a threat to their own since Veda as it was the only religious script written in Sanskrit. However, after the translation of the Mahayana scriptures into Sanskrit by the Mahayanists, the Brahmins started reading these translated texts and to argue pointing out its content.

In trying to defeat these Brahmins, the Mahayanists built large universities like Nalanda with the aim of learning the Veda. The language used in these universities was Sanskrit. They tried to outdo the Brahmins. They tried to argue with the Brahmins. Thus what came to pass was exactly as mentioned in the Alagaddūpama Sutta, which states that “if anyone tries to learn my teachings to argue with others, then he will be destroyed like the man who tried to catch a snake by its tail.”

When the monks won an argument the monks got the king to make the Brahmins their servants in the Nalanda University. The monks were beheaded when Brahmins won their arguments. The Brahmins went for the kill, because of the change that these monks have made to the Brahmin society. A Sothern Indian Brahmin called Sankarāchārya rose up to destroy the power of these monks. The Musālman invaded India at this time. The Brahmins got them to first destroy Buddhism, or at least what was left of it. This was how the Buddhism got wiped out from India.

It was the moguls that did most of the killing of Buddhist Monks, and the process of decline continued until Buddhism got fully wiped out in India by the end of the 12th century, when Somapura Mahavihara (in the present day Bangladesh) was set ablaze. In 1199 CE Odantapuri Mahavihara close to Nalanda was burnt down after killing all the monks. Buddha’s place of Enlightenment, Bodhgaya too was attacked.