[ThirdSight]

I'd imagine it's all in the marketing. My facts are most likely wrong, but the PS3 got off to a pretty bad start, losing money over the fact that their system cost more than the games did; it'd take a long time for a gamer to pay his system off in games he/she enjoyed. 360 on the other hand, made it's way into the market as the newest next-gen system before any other system did, and it's got a good lead as far as number of games are concerned for it. A lot of people own it, though PS3 has made it's way into people's homes with MGS4 and a few others. Of course, the PS3 only has a few "good" games for it, whereas the 360 has a fairly wide selection to choose from.

SE may just see that it'd reduce the cost to put their game on the 360, and they'd turn a larger profit from it instead of having it exclusively on the PS3. Of course, SE doesn't want to lose the audience that they have already, so they're releasing it for both systems instead of making an entire switch.

At least that's my take on it.