To understand the scenario, you need to know the history of North eastern states of our India.
When India got its independence in 1947, northeast comprised of three states- Assam and the princely states of Manipur and Tripura. (Like you had wished)
As you can see, Assam was much bigger than present day Assam and included many tribes like nagas, garo, khasi, jaintia etc. In 1960, Assam declared Assamese as the state language. This move led to revolt from the different linguistic people who lived in Assam. It became very difficult to control such a big and diverse Assam.
Subsequently, Nagaland was formed in 1963 to please the violent Naga people who had taken arms against the army, Meghalaya in 1972, Arunachal Pradesh in 1975 and Mizoram in 1987 where Indian government had to use air Force to quell an armed uprising of Mizos!!
So, reasons like linguistic convenience, armed rebellion of tribes etc. were some reasons which led to division of Assam.
Union will again lead to disorder and unrest in northeast which could be exploited by our ‘well wishing' neighbors.
