There are probably a couple of reasons:

1. The total number of startups is going to be lesser due to redtape although I believe that is becoming less of an issue as time goes on.

2. It is probably perceived to be more lucrative to target the American market. I would think that a larger proportion of the ~ 120M internet users in India would be similar (young urban professional) and therefore easier to release products compared to the general public (cultural variations and interests make it harder to disrupt outside the cities).