or maybe ps3 is just much less successful as ps2 and people who cared about those kind of games have no console anymore which means those games would also sell less on ps3 if they wouldn't be also on 360? (as example ff)
not to talk about dying genres like fighting games which not many care about nowadays. (there is a reason why people call this gen the shooter gen)
i hope that's the reason because otherwise people are just stupid if they stop buying games they like just because they aren't exclusive anymore.
btw how much did san andreas sell on ps2 before it was released on xbox? because from then it wasn't exclusive anymore and i remember it sold a ton even years after release.