with assam the reason is clear cut. assam produces 90% of india's oil and yet the 2 biggest indian companies - indian oil and ONGC are headquartered in delhi and derhadun respectively and not in assam. for all indian psu's the head office in the place that areally makes the psu happen and the maximum employment is also given to people of that state. plus the way nehru dumped the assamese after lagao-ing the china war was a blunder only a jackass like nehru coult commit. with the other NE states, its because of geopraphical isolation and cultural difference.
places like nagaland never really belonged to india - the brits annexed it to raj india after wresting it from the burmese and and the local naga population. when they left, nagaland fell into indian hands and ever since new delhi has done precious little to reach out.
tripura actually always belonged to united bengal but now wallows on the other side of bangladesh. the local people of tripura are not mongoloid.
with manipur and meghalays, india has had ties with them since mahabharat days. the manupuri dance is based on opne of the chapters of the mahabharat. the very name meghalaya = megh + alaya = abode of clouds, in sanskrit.
i do not know much about mizoram and arunachal or their history.
"Take my hand, my child of love
Come step inside my tears
Swim the magic ocean,
I've been crying all these years"
"I liked the world where i used to live
I never really wanted to live"