please mark brainliest
You might have mentioned under which parameters you consider it as neglected. Anyway, lets see few scenarios:

Population: Actually, number of Lok Sabha seats depend upon population density. So for the same reason North-East Indian states are not much represented with a good number in Lok Sabha by the politicians as they are not densely populated when compared with the remaining states, but this doesn't mean that these regions are neglected. They are represented proportionally. Period! 

Business: The climatic conditions decides the commercialization and connectivity within India. Hilly terrains and rainy climatic conditions won't let the big shots to establish their businesses in the North-East. Do I need to mention about the priorities to launch a business? So the North-East Indian states contribution is less in terms of GDP. This makes them feel inferior, but they are not!

Attraction: But trust me, our North-East is a paradise. People usually call East India a Seven-Sisters paradise (Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland and Tripura) and the crowd are civilzed. Stressed this because, few presumed that they look uncivilized. Yes, the people, their cultures and traditions are attractive. But regular floods is one such heavy reason behind withdrawing the support. 

Setbacks: These regions seriously lack national media coverage, attractive infrastructure to attract the capital towards these regions both politically and big entrepreneurs as well, poor power generation and many more . So this lacks job opportunities and people started protesting against the government. Again this discourages the business people to launch new initiatives in these regions. You can observe these kind of agitations on news. Not to mention about Bangladesh migrants.

Cinemas: The festivals and celebrations in the North-Eastern states of India are a colorful reflection of the people and their lives, but they are not projected well onto the cinemas or books. I do know about this region as I have to study about them, but many of my mates don' know about their cultures, life-styles, beer fests, tribes and many more. We do know well about North and South Indian states. (Of course because of Indian Cinemas :P)

So, they are not intentionally neglected but there are different reasons for each of it. But the word 'neglected' is much overrated than the actual problem.
hope it helps u