Why is Buddhism followed mainly outside of India when it originated there?

Every inspired leader lives spontaneously in their life and instills a fresh experience of awakening to any who are ready to be broken open so to speak to this moment. What happens after they are gone, too often, is a division into antagonistic camps. This can squash the original innocence of awakening. When Bodhidharma brought the buddhist awakening to China it found a fresh environment ready for catching on and flourished. Dogen brought the dharma to Japan but was rebuffed initially and lived far away in the mountains until the hungry dharma seekers came to him. https://www.youtube.com/watch?v=TinmRC2BS00 The history of Dogen is a fascinating 2 hour long movie not about him but acted out as he and his followers are believed to have lived. What really determines where an idea is stopped is where it crashes into the power grid of its day. And then sometimes it transmutes again. Is that we are now?

Note:(1) This is a very controversial topic in India and Indian diaspora and India is ruled currently by Far- Right with a huge voter base and a lot of times in the history of India far- right people has been at the helm of the nation in bureucracy, politics and in Academia.(2) So, I request the Moderators to protect the answer from defiling by rouges.(3) Any bad comment or comments in poor taste will not be replied to.

"How can we explain the fall of Buddhism in India, which is the geographic origin of Buddhism?

Were there ever any direct conflicts between Hinduism and Buddhism?"

Brahmin caste and their supporting castes were responsible for the fall of Buddhism in India. I will support my this argument with sources from History and Philosophy books.

Excerpts from the book Debrahmanising History, by Braj Ranjan Mani(https://books.google.com/books/about/Debrahmanising_History.html?id=hjxuAAAAMAAJ#:~:text=Debrahmanising%20History%20Is%20A%20Sweeping,History%2C%20From%20Buddha%20To%20Ambedkar.) , Chapter 4 , Chapter Name: Buddhist India : The mauryans were not interested in giving brahmans preferential treatment and certain immunities that they demanded as their birth right. Ashoka introduced the policy of equality in civil law and criminal law for everyone( Page 129-130). In another edict, Ashoka exposed the Bhudevas ( gods on Earth ) as false gods. In Ashoka's rule there was indifference to Sanskrit, the language of Brahmanic Priestcraft. Such a policy hit the interests of the brahmins who depended on the donations and gifts given to them in various rites and sacrifices. In other words, the superiority of the brahmans and the credibility of the sacerdotal literature were under threat.

The Brahmins retaliated . Led by Pushyamitra Sunga, the commander of the last mauryan king Bridhadratha, they hatched a conspiracy. Pushyamitra gave his daughter in marriage to the king, got coveted post and cold - bloodedly beheaded his master during a military parade in 185 BCE... After burning monastries in and around Patliputra he went up to Sakala and offered a reward of 100 dinars or gold pieces for the head of every Buddhist monk.... That he unleashed a reign of terror against buddhists is corroborated by several sources including the Tibetan and Chinese records. From Chinese authorities it is known that many Buddhists still do not pronounce the name of Pushyamitra without a curse( cited in Ambedkar, ibid). Rejecting the established trajectory of Indian history, Ambedkar in his incomplete work The Revolution and Counter Revolution in Ancient India has made the conflict between the Buddhism and Brahminism the corner stone of restructuring the past. ( K.P. Jayswal , too in his Manu and yajnavalkya [1930: 40-1], has termed the brahmanical reaction the 'orthodox counter -revolution'). Ambedkar's contention that the process of the decline of Buddhism in India was a violent one ( not a process of gradual adjustment and absorption into Hinduism as is routinely potrayed by the elite historians ) has been corroborated by ample archaeological and textual evidiences in Verardi's brilliant work. (2011) ( Page 130- 131).

Ambedkar says: " Now if one bears in mind the fact that the Brahmanic Revolution by Pushyamitra took place in 185 BCE there remains no doubt that the code known as Manusmriti was promuglated by Pushyamitra as embodying the principles of Brahmanic Revolution"

Page 132: Ambedkar cites some verses from the Manusmriti to show the author's antipathy to Buddhism.... Ambedkar sees parallel between the Muslim invasions of Hindu India" and brahmanic invasions of " Buddhist India". ..., the brahmanic invaders of Buddhist India, the Shungas, the Kanvas and the Andhras fought among themselves for superiority but had one common goal to destroy buddhism. ...

In his analysis of destruction of Buddhism , Ambedkar also mentions a "subversion theory". ... the subversion involved the insidious role of brahman - Buddhists. ... The higher philosophies of both Buddhism and brahmin began to converge in essence. ... In other words , they continued to be brahmins despite accepting buddhism and probably observed their caste excluvism as well. They produced doctrines akin to the metaphisyics of brahmanism which culminated in the Mahayana school...

Page 133: The Mahayanists transformed the Buddha into a god, introduced his several incarnations. ... even tried to co-opt Buddha in the brahmanical mould by depicting him as the ninth avatar of Vishnu.

Similar research is also done by Swami Dharma Teertha in his book History of Hindu Imperialism:https://www.goodreads.com/book/show/54301472-history-of-hindu-imperialism

Kings like Harsh Vardhan , Kansihka who supported Buddhism were killed by Brahmins and their supporting castes by hatching conspiracies.( Source: the above mentioned book by Swami Dharma Teertha).

One of the main reasons for the decline of Buddhism in India is that it was engulfed by Hinduism in the years following the death of Buddha.

India, before the time of Buddha was a Brahminical society dominated by Brahmins and Kshatriyas who were at the top of a four level caste system, the others being Vaishyas followed by Shudras. The Indian society was rife with rites and rituals which could be only performed by Brahmins. A householder throughout his existence was supposed to carry out various sacrifices which included animal slaughter and thus the vasihyas and the shudras suffered the most during pre-Buddha period. Now Buddha's teachings gave no value to rites and rituals and also he preached non-violence. This made Buddhism much popular among the vaishyas and shudras. This was of course a threat to Brahminical dominance and thus the livelihood of the priestly class. Hence there were many attempts by the priestly class to sabotage Buddhist teachings even during the time of Buddha but they were unsuccessful.

After his death, they seem to have found success. Although Buddha negated the existence of God as is accepted in Hinduism, later Hindu fanatics made out Buddha himself to be a Hindu God. They declared him to be the 9th avatar of Vishnu, a Hindu God.

Buddhism declined from India in the past, due to assimilation.

Firstly, Buddhism had flourished in ancient India, Pakistan and Afghanistan, but eventually, the Buddhist locals became subject to "hard" assimilation by foreign invaders. See the Wikipedia articles Buddhism in Afghanistan, Buddhism in Pakistan and also this article section. This is not unlike the disappearance of Zoroastrianism from its homeland of Persia.

Secondly, Buddhism suffered from "soft" assimilation into Hinduism.

Before Buddhism (around 500 BCE), the historic Vedic religion was mainly focused on sacrifices and rituals. It did not have robust philosophy and metaphysics. After encountering Buddhism and the shramanic movement (which includes Jainism), the Upanishadic movement rose in India from 500 - 200 BCE. After this, the Vedanta movement rose as part of Classical Hinduism from 200 BCE - 1100 CE. The Hindu text Brahma Sutras that attempted to refute Buddhist and other non-Hindu teachings, was written between 200 BCE and 400 CE. See this question for details.

Adi Shankara, the famous reformer who revived Hinduism and consolidated Advaita Vedanta, lived around 800 CE. Advaita is the philosophical school of Vedanta that is closest to Buddhism. Even in those days, Hindu philosophical opponents of Shankara accused of him of being a crypto-Buddhist. Through Advaita Vedanta, Hinduism assimilated Buddhism by adapting parts of its philosophy unto itself. Some Hindus even claim that Mahayana's emptiness (shunyata) is exactly the same as Advaita's Transcendental Ultimate Reality. See this answer for details.

On the other hand, the Dvaita Vedanta school, which has completely incompatible philosophy to Buddhism, assimilated Buddhism by making Buddha an incarnation of the Hindu deity Vishnu. The Hindu text Bhagavata Purana written between 500 CE and 1000 CE states that Vishnu incarnated as Buddha to delude the immoral atheists into living ethical lives. According to them, the Buddha tricked atheists into taking refuge in the Buddha and serving the Buddha, who is actually God in disguise. More info on this is available on this page.

Hindu philosophers also claimed that Brahma Sutras, Bhagavata Purana and other such texts were divinely authored a long time before the arrival of the Buddha. This was one way to convince Buddhists that Vishnu's incarnation as the Buddha was foretold by the Hindu scriptures even before his birth. This was debunked by professional academicians.

In my personal opinion (which may lean towards Theravada), the final reason for Buddhism's "soft" assimilation into Hinduism is the development of concepts in Mahayana Buddhism that sound very similar to concepts in Hinduism, like the Eternal Buddha, Adi Buddha (Primordial Buddha), Buddha Nature (tathagatagarbha), storehouse consciousness (alayavijnana) and beings like Avalokiteshvara etc.

In his book "Indian Philosophy: A Critical Survey" on pages 63 - 64, Professor Chandradhar Sharma wrote very misleadingly about Buddhism:

Buddha is here transformed (by Mahayana Buddhism) into God and worshipped as such. He is identified with transcendental reality and is said to possess the power of reincarnation. The Buddha is the Absolute Self running through all the so-called individual selves. He is the Noumenon behind all phenomena. ....

Dry asceticism of the Hinayana is replaced by an enlightened and loving interest in this world. The nega­tive and individual conception of Nirvana is replaced by the positively blissful and universal conception of it. The denial of God is replaced with the Buddha’s Divinity. The explosion of matter and mind by reducing them to the series of momentary atoms and momentary ideas respectively, is replaced by the admission of the relative reality of both, the transcendental reality being the Absolute, the Luminous Body of the Buddha. The greatness of the Mahayana lies in its spirit of selfless service of humanity, its accommodating spirit and its missionary zeal. The Mahayanists are reasonably proud of their faith as a progressive and dynamic religion which throbs with vitality because it has the capacity to adapt itself with the changing environmental conditions, preserving its essentials intact. .....

It (Hinayana schools) has ignored some important implications in the teachings of the Master and has misinterpreted many. Some of the main doctrines of the Buddha have been taken to their reductio ad absurdum pitch. As a matter of fact, there was nothing in the teachings of the Buddha which would seriously militate against the Upanisads. The Lamp of Dharma bequeathed by the Buddha to his disciples was borrowed from the Upanisads. But the Hinayana made the constant and the luminous light of this Lamp flickering and faint.

This was refuted by Banaras Hindu University Professor T. R. V. Murti's statement (quoted below) in "Problem of Two Truths in Buddhism and Vedanta" (page 9):

It has been the fashion to consider that the differences between the Madhyamika śūnyatā and Brahman are rather superficial and even verbal, and that the two systems of philosophy are almost identical. At least Professor Radhakrishnan thinks so, and Stcherbatsky's and Dasgupta's views are not very different. I hold a contrary view altogether: that in spite of superficial similarities in form and terminology, the differences between them are deep and pervasive.

This was also refuted by eminent German indologist Professor Helmuth von Glasenapp's essay "Vedanta and Buddhism: A Comparative Study":

Nothing shows better the great distance that separates the Vedanta and the teachings of the Buddha, than the fact that the two principal concepts of Upanishadic wisdom, Atman and Brahman, do not appear anywhere in the Buddhist texts, with the clear and distinct meaning of a "primordial ground of the world, core of existence, ens realissimum (true substance)," or similarly.

Buddhism survived in the world through three routes - Tibetan Buddhism (and the preservation of the Tibetan Buddhist canon), East Asia (through transmission of Mahayana texts to China) and Theravada Buddhism's revival through Buddhism's own version of Adi Shankara, Buddhaghosa.

Buddhism has experienced a limited revival in India due to three reasons. The first is the conversion of low caste Dalits to Buddhism by Dr. Ambedkar (see this question). The second is the popularization of Vipassana meditation by S. N. Goenka. The third is the relocation of the Dalai Lama to Dharamsala, India.

I think the invasions of India cannot be a sufficient reason for the decline of Buddhism. Much more is needed to destroy a religion than the destruction of holy sites (think about the followers!). Apart from that, Hinduism must have been at least as offensive to the piety of Muslim invaders - if not more offensive for obvious polytheistic idolatry - than Buddhism must have been, while Hinduism was not at all destroyed in India by the invaders. Moreover, the conquest of the south of the subcontinent could not happen before the fall of the religiously tolerant Vijayanagara Empire in the 17th century. I think the shortcomings of a monocausal theory of Muslim invaders destroying has been sufficiently shown.

For a time stretching from shortly after the Buddhas Nirvana to at the very least the beginning of the Common Era, Buddhism had been the most vigorous social, intellectual and religious movement of India (referring here to the Indian civilization, not to a state-entity). This flourishing opened out into a long decline paired with a "renaissance" of Hinduism (or rather: establishment of Hinduism) and Sanskrit learning paired with an increasingly powerful position of Brahmins in the political architecture of the Indian states. The reasons for this are rather complicated, you can get a not too much in-depth glance of them here: http://en.wikipedia.org/wiki/Decline_of_Buddhism_in_India while if you want to know more in depth, check the sources given there or (maybe) any other academic source on history of Buddhism.

India (the modern country) didn't exist when the Buddha was born.

The Buddha was born near modern-day Northern India or Nepal.

Wikipedia's Decline of Buddhism in the Indian subcontinent says,

[This] refers to a gradual process of dwindling and replacement of Buddhism in India, which ended around the 12th century. According to Lars Fogelin, this was "not a singular event, with a singular cause; it was a centuries-long process."

The decline of Buddhism has been attributed to various factors, especially the regionalisation of India after the end of the Gupta Empire (320–650 CE), which led to the loss of patronage and donations as Indian dynasties turned to the services of Hindu Brahmins. Another factor were invasions of north India by various groups such as Huns, Turco-mongols and Persians and subsequent destruction of Buddhist institutions such as Nalanda and religious persecutions. Religious competition with Hinduism and later Islam were also important factors.

This History of the Delhi Sultanates says,

Between 1160 and 1187, Muhammad al-Ghuri conquered the Ghaznavid Empire; he continued to expand his territory, and in 1192 he razed the Buddhist temple complex at Nalanda to the ground, in effect terminating the history of Buddhism in India.

It references a student's paper on the The History of Persecution of the Buddhist Faith, which includes,

Among the destroyed temples, 2/3 were Buddhists, and this was critical to Buddhism's ability to remain as a formal religion because they had already lost all but a few institutions in Nalanda, Odantapuri, and Vikramasila, over the previous centuries. (20)

The Nalanda University, a great Buddhist center of learning, was raided by Turkic Muslim invaders under Bakhtiyar Khalji, a general of the Turkish commander Qutb-ud-din Aybak, in 1193. He committed documented executions, harassed and tortured erudite monks, killing 15,000 scholars and 200 faculty of the University. (21) The campus and invaluable works of art including the images of the Buddha were destroyed and the enormous manuscript library of the University was burned down. He also destroyed the monastries in Vikramshila, which were in modern Bihar, as well as many monastries in Odantapuri in 1197. As he persecuted Buddhism, he supported Muslim missionaries and made the biggest number of converts to Islam under his reign. (22) By the end of the 12th century, many Buddhist monks retreated to Nepal, Sikkim, Tibet and Southern India. (23)

Persecution of Buddhism was accelerated in this period by Brahmin revivalists who kicked the Buddhist monks out of Buddhist monasteries and temples in order to transform the places into Hindu institutions; they were seeking protection from Muslim invasions by facilitating the installation of Brahmin gods. (24) No less than 1000 Buddhist temples were appropriated by Hindus, etc.

The (numbers) in the quote above are references to various history books etc.

There's a Wikipedia article (whose neutrality is currently disputed) titled Persecution of Buddhists which includes,

In 1200 Muhammad Khilji, one of Qutb-ud-Din's generals destroyed monasteries fortified by the Sena armies, such as the one at Vikramshila. Many monuments of ancient Indian civilization were destroyed by the invading armies, including Buddhist sanctuaries[45] near Benares. Buddhist monks who escaped the massacre fled to Nepal, Tibet and South India.[46]

... and continues in a similar way.

Perhaps these are the reasons: i.e. they're historical reasons.

It might seem as if it would be interesting to try to answer this question by discussing 'motive' (e.g. "why would people prefer to be Hindu than to be Buddhist"), comparative religious views (e.g. about 'just war'; and about how well the religion is integrated into the broader society at large), and comparative values (which religion is more useful, more attractive, more true) but I expect that would be subjective opinion, off-topic and unhelpful.

I'd also mention this answer to the question, Were Buddhists murdered en masse during the first Muslim conquest of India?

It references a thesis which says that Buddhism declined (not especially violently) during the 200 years or so following the first Muslim invasion -- i.e. in the 8th century AD -- mostly because urban merchants (who previously supported Buddhist monks) converted to Islam for various reasons; whereas the more-Hindu farmers were relatively unaffected.