During the period around Independence, India, under Sardar Vallabh Bhai Patel's program to unite the subcontinent, was getting Princely states to accede to Union of India. There were around 570 Princely States. Then why were Nepal and Bhutan, considered, and recognised sovereign nations, and not just another princely state?
Modern India evolved out of the transitionary Dominion of India, which was created from territories of the British Raj. It is important to note that neither Bhutan nor Nepal were princely states under British India. In Nepal's case, the Himalayan kingdom successfully negotiated a Treaty of Friendship in 1923, in which Britain recognised Nepalese sovereignty.
Article 1. There shall be perpetual peace and friendship between the Governments of Great Britain and Nepal, and the two Governments agree mutually to acknowledge and respect each other's independence, both internal and external.
Likewise, Bhutan was not a princely state; however, its status suffered from a policy of "convenient ambiguity" by British officials. In a 1924 decision, the Government of India resolved that while Bhutan was under British suzerainty, "it is doubtful whether it can be regarded as an Indian State, and it is expedient to leave its status ambiguous as it is undesirable to extend British commitments in regards to Bhutan". This remained the British position until Indian independence.
The issue of Bhutan's status vis-a-vis the government of India (was Bhutan a state of India or did it enjoy internal sovereignty?) was re-examined by London in 1932 as part of the issue of the status of India itself. It was decided to leave the decision to join an Indian federation up to Bhutan when the time came.
- Bisht, Ramesh Chandra. International Encyclopaedia Of Himalayas (5 Vols. Set). Mittal Publications, 2008.
Since it was left deliberately vague, Bhutan's pre-existing independence was maintained. Technically it never did become a princely state under the British Raj.
The British accepted the following criteria for a native princely state: (i) the right to recognise succession and to regulate disputed successions, (ii) the right of intervention prevent dismemberment of a state, (iii) to suppress rebellion against the lawful sovereigns, (iv) to prevent gross misrule, (v) to check human practices or offences against natural law or public morality, and (vi) to secure religious tolerance. However, the British had never claimed and exercised the above rights in Bhutan and, in fact left it to itself so far as its internal affairs were concerned.
- Sinha, Awadhesh Coomar. Himalayan Kingdom Bhutan: Tradition, Transition, and Transformation. Indus Publishing, 2001.
This legal picture had not changed by the time of Indian independence, when Sir Arthur Hopkison, the last British Political Officer of Sikkim, Bhutan and Tibet, observed in a communication to the Government of India that: "Bhutan fears that it may be confronted with some decision classifying it as an Indian State, which it is not, and lumping it along with other Indian states".
The British Act of Parliament that established Indian Independence the next year states:
The territories of India shall be the territories under the sovereignty of His Majesty which, immediately before the appointed day, were included in British India.
Therefore, on a legal level, this excluded Bhutan and Nepal.
After independence, the Union of India wanted to maintain the Himalayan frontier policy of British India against an ascendant Communist China. Bhutan and Nepal (and Sikkim) were strategic buffer states for this purpose. For their part, the Himalayan kingdoms chose to maintain and strengthen their link to India while manoeuvring to maintain their independence. Bhutan reoconised Indian suzerainity in a 1949 treaty, in which it agreed to be "guided" by gave India in foreign affairs. Nepal also signed a Treaty of Peace and Friendship in 1950.
These measures more or less eliminated any immediate need by India to fully annex the two kingdoms. Rather than potentially upset the regional balance of power, as well as risk additional internal destabilisation with unnecessary annexations (especially given Bhutan's Buddhist population), the statue quo pre-independence was maintained. To a certain degree this outcome might be attributed to India's good will, but pre-existing legal conditions and strategic considerations were important factors.
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.
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.
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.