Linguistic diversity is like biodiversity… It increases the closer you are to the equator. Mountains and jungles are also hotbeds.
We don’t know how many of the languages in India got to be there… Successive invasions introduced new languages but they were never entirely successful in wiping out the native ones.
There’s an obvious (and long-standing) question about the impact of technologies that overcome geographical limitations – both transport and telecommunications technologies. It’s the reason Chinese is feeling pressure from English within its borders (witness it taking steps to limit Chinglish).
So far I’ve not found much on the impact on language diversity of mobile phone technology, which is now rolling out most quickly and gaining deep penetration in precisely those areas which have traditionally preserved languages: remoter rural parts of Africa and the Indian subcontinent.
Update: It seems the Long Now foundation are thinking about mobile (of course) – see this piece.