In 1947 when India was given freedom by the British it was thought a new dawn of development and prosperity would be launched. However Nehru with his zeal to copy the British carried on the age old British policies as far as the tribal lands were concerned.
The British who were the paramount power and wished to further their rule had no interest in integrating the tribal areas with the rest of India.Thus they formulated policies where the tribals were left to themselves with only a loose form of government control. So much so that to visit the tribal areas of Assam and Nagaland an inner line permit was required.They effectively insulated the tribal areas from the rest of the country.
The Indian Government under Nehru continued with these British policies even after 1947 when independence was granted. Thus the inner line permits remained and very little effort was made to integrate these areas with the mother country. It did not help as racially these tribals were not from the main Arayan or Dravidian stock.