Note: I asked this Question in Quora and i got the following Answer
It is not that the north-eastern states were not conquerred. There are many times and conquerred them. For example the Mughals, the Burmese etc. It is just that they couldn't enforce their rule owing to a number of common reasons.
1. The people: The various tribes and people of north east are very much soverein minded. They would not take foreign rule.
2. Guerrila warfare: North east tarrains by nature are very suitable for guerila fight, the foreighers usually were not accustomed to it.
3. The summars and rainy season: Especially the rainy seasons in North-East were notorious for rain, flood and spreading various epidemics like cholera, malaria etc. Foreighn soldires had less immunity to these compared to the natives. This alone were the reason for many foreign armies to pull away.
These are general reasons.
Nabanshu Bhattacharjee :I think lack of interest is also important. Tax on agriculture used to be the most important component of revenue. Subsistence agriculture of the hilly states would have definitely been pretty unattractive
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