India has not joined the Nuclear Nonproliferation Treaty because, well...
From early on in the development of the treaty, India objected to the two-tier nature of it: five acceptable nuclear states, who just happen to be the permanent members of the United Nations Security Council, and no nukes for anyone else. There are at least two ways to look at that disparity: stopping the spread of nuclear weapons where it was in the early 1960s, when the treaty was being developed, or that if some have the right to nuclear weapons, all must have the right to them.
Indian rhetoric slides from one to the other, as does the rhetoric of some who claim to be for no nukes. Indian reality is that it has a fully functioning nuclear weapons industry that has turned out something in the vicinity of a hundred nuclear weapons.
India has some of the geographical characteristics of an island: protective ocean to the south and protective mountains to the north, but unfriendly Pakistan to the west and needy Bangladesh to the east. This is somewhat like the United States: protective oceans to the east and west, and reasonably friendly Canada and Mexico at the other borders.
India has been unhappy with being excluded from nuclear trade by its choice to remain outside the NPT, and the Bush adminstration decided that India was a valuable enough trade partner (for jet aircraft and other military appurtenances, anyway) that it would lift that exclusion. Its rhetoric slid from encouraging India in its nonproliferation duties to improving the economics of the US nuclear industry. The reality of the agreement reached was very little additional safeguarding of India's nuclear establishment and opening up India's nuclear trade to all.
India's internal politics almost scuttled the deal the first time around; nationalistic elements wanted less scrutiny and more control over trade than might reasonably be expected. Now internal politics have again triumphed over the realities of international trade. India's parliament has passed a bill requiring that foreign firms working in India assume higher levels of liability than most companies are willing to.
That parliamentary concern arises from India's experience with Union Carbide's chemical disaster at Bhopal in 1984.
There are other issues, too, that India would like settled to its preferences while offering nothing in return. A Newsweek article summarizes them in advance of President Obama's visit to India in November.
We might consider, in that other almost-island nation, the overreactions to the events of September 11, 2001, and how the congress of that nation continues to work its internal concerns, ignoring how their actions may look to the rest of the world or how they may affect relations with others.
The tone of the Newsweek article partly echoes that insularity of the nation for which it is primarily written: India must pull up its socks!
The difference between the two nations, of course, is money and power. India has been steadfast in its views for quite some time.