From what I understand, at the time of Partition and Independence of India and Pakistan there were around 500 Princely States in existence. The British gave these states a choice: either accede to India or Pakistan or remain an Independent State. Surprisingly none of the States decided to remain Independent. Within two years all of the states except Sikkim had acceded.
The question is, why did all the rulers of the Princely State decided to give up their power, title and independence and instead chose to accede?
I can't answer your question, unfortunately, but it's worth pointing out that not all the princely rulers gave up power without a fight - the Indian Army was forced to conquer Hyderabad in 1948 when its Nizam refused to join the new Indian state.
https://en.wikipedia.org/wiki/Indian_integration_of_Hyderabad