Why did Nepal and Bhutan not become a part of India like other princely states when colonial rule ended?

History has its place in shaping the borders of countries. So Nepal and India became separate countries. However, changes in time and potential opportunities and threats may force countries to rethink on every thing including borders. If there is any country with which Nepal and their people can think of shared traditions, culture , food habits and history, it is with India. Hinduism and the practices of worship make both people understand and relate to each other. In such situation, why not to unite and work together for a strong and shared future? Indians do not mind in sharing the wealth they created with Nepalis as Hindus consider Nepalis as a part of their own. Recently a neighboring country of USA by name Georgia, passed a resolution in their parliament , to merge with USA as 51st state. It is a border country and they saw, security and welfare in being part of USA rather than staying as a neighbor depriving itself of all the opportunities a combined country offers. Is the case of Nepal not similar?

Simple reason: these areas were never conquered by the British.

The Anglo-Nepalese War ended with the Treaty of Sughauli, with the ongoing colonial relationship being the recruitment of Gurkha soldiers to the Indian and British Armies.

We must not forget that the only factor holding the British Raj together was that it was the area colonised by the British in South Asia. There was very little else in common (culturally, linguistically or ethnically) between the Hindu Tamil from Cape Comorin and the Muslim Pashtun from the North West Frontier Province. The Partition meant that the definition of India was the area colonised by the British with a Hindu/Buddhist majority. Hence we see issues in Kashmir and some Christian tribal majority North East states, because of the clash with the so-called "cultural narrative".

It's almost unfathomable to imagine that Nepal would now give up its sovereignty and joint India, to adopt Hindi as an official language, the signs and symbols of the Indian Union- such as Bengali Jana Gana Mana and the tricolour flag; as well as parliamentary and legal systems that have developed over 150 years in India.

If anything, the unquestioned independence of Nepal is proof that if the British had never united India, it would not exist as it currently does right now.

We should keep in mind during Delhi Darbar in 1911 Nepal was given seat with the group of independent nations like Afghanistan. Nepal refused to remain among at the first position of princely states.

There were several reasons while Nepal, Bhutan, (and until recently, Sikkim), remained Independent of India.

1) The first was their remote, mountainous locations. That made it hard for Britain to occupy them, and the fierce soldiers of Nepal (the Ghurkas) were particularly prized by Britain. Basically, it was easier and more profitable for Britain to "deal" with them than conquer them.

2) The second reason (related to the first) was the power and sway of local warlords in these countries that made for the issues in 1) above; easy cooperation, hard conquest.

Sikkim, the smallest of the three, was nominally independent until 1975. Its affairs had been managed by India but they maintained "administrative autonomy until civil strife brought them into the hands of India.

Firstly the "princely state" is the term given to the states directly controlled by the British during the British Raj due to conceding freedom for one reason or the other. Bhutan like mentioned in above answer was of no interest to the British and they decided to keep it's state ambiguous. However Nepal under the Shah Kings had gained significant military power. Successful military campaign under Bahadur Shah meant that Nepal's territory in the west had reached Kangra and Sikkimm in the East. Nepal did quickly withdraw from Kangra and Sikkimm. But Sikkimm continued to pay tax to Nepal. In the south as well Nepal has great success stretching it's borders through successful military campaigns. Nepal's borders reached as far as Gorakhpur.

Nepal's the military conquest seriously threatened the British intention in India even though the British Raj had not started still. Thus the Anglo-Nepal began in 1814. Nepal initially had some success in the war but consequently had to sign a treaty that meant she lost lot of territory to the British but gained recognition of Sovereignty from the British which meant that it successfully kept itself above the status of any other state in the Indian sub-continent as per the British. Noticing the military capabilities of the Nepalese army the British since then started hiring Nepali mercenaries now famous as the Gurkhas.

In later years Nepal was an ally to British and more than once helped the British suppress rebellion. Most notable was the crushing of The Indian Rebellion of 1857 where Nepal sent an army of 15,000 to aid the British in Gorakhpur and Lucknow. Just before that Priminister of Nepal Jung Bahadur Rana had visited Britain where he was saluted as a leader of a Sovereign nation. Even till the later years Kings and Heads of Nation of Nepal have received the highest respect upon their arrival to Britain. For example the visit of King Mahendra to Britain in 1960.

All this clearly shows that Nepal has its status above any other princely state of India. Any question asking why Nepal is not part of India after withdrawal from British is like asking why India is not part of China or why France is not part of Britain. Before even India won its Independence from Britain, Nepal was already an important ally to Britain equal to its other allies.