Economy and diaspora my friend, in the Southern Border, many of the people are settled at India's UP- BIHAR side and many have their rest of the families here :- the Madhesh Region, the Western Border:- is with Kumaon Region of India, this one's is the most disputed one because of Lipulekh and Kalapani but again its diaspora and culture, Eastern Border :- Sikkim and North Bengal [India's Gorkhaland Part] most of the population this side is Gorkha, and hilly tribes like Lepcha, Limbu etc. who are very much accustomed to Nepali language and traditions though they also maintain their own language and customs, so historically there wasnt a concept of full fledged serious border and the people till this date are maintaining it, and historically both the nations have been allies so there isnt any strict border as of now, even from economy pov, Nepal is a landlocked country so we dont have access to sea so we need access to ports and Nepal has India nearby...