It's simple: price.
A 360 can be bought for $200 and a ps3 cost $400.
Once the ps3 gets a price drop it will probably be outselling the 360. It would also help the ps3 to get some JRPG's in the US because it is completely missing out on fans of that genre. JRPG fans in the US have no reason to buy a ps3 at this point.
PSN ID: Sorrow880
Gamertag: Sorrow80
Wii #: 8132 1076 3416 7450