Regarding the affording thing , India is the second largest military equipment importer after China in the world.
Second as someone said US has shortlisted countries whom they will sell their stuff to, recently India got an offer from Lockheed Martin to sell their F-16 jets.But India rejected it as the pakis already have it plus recently their was a joint Indo-Singapore air combat excercise and singaporeans had F-16's with them and the Russian su-30MKI jets proved to superior to them..that might be one reason.
And I think theres not much good relations between India and US when it comes to selling military euip. might be because India gets stuff from Russia and therefore USA has problems or maybe India sees Russia as a stronger and a steadfast ally than USA.
Also I think that the F/A-22 Raptors should be tested against Sukhoi's, maybe India is waiting for that ??!!
I also read some news that India is gonna produce Su-30 MKI's indigenously starting next year.
But the main reason I think is USA doesn't want to disappoint paki+satan
by selling high tech stuff to India.Once the so called war against terror is over USA is gonna dump Pakistan, pakis don't have an idea or do they ?