I believe there are still far more PS2 games playable on the PS3 than there are Xbox games playable on the 360.

There are also a lot more ps2 games than original Xbox games. It would be interesting to see a % comparison.

I think its just different views of the companies that drove their backcompat efforts. Microsoft with only their second console wants to make sure that every game they port works, while Sony knows economically thats not possible with the large title library, so they say 'here ya go, if it works good for us. if not, suck it up'.