A couple of reasons I think. First of all, the Xbox has an advantage over the PS3 when it comes to WRPGs, and they sell better on the Xbox. Second, BioWare is much more widely known among the Xbox community than it is among the Playstation community. There were probably a fair number of people whose number one reason for buying the game was because BioWare's name's on the box, they really have a dedicated following, and since their major releases of the past six years have all been on Xbox, it gives it an advantage there. Squilliam's right too with his other examples, and I'll point out all of those developers released a title on the original Xbox while none of them did on the PS2, which is another reason why WRPGs are more popular on Xbox.
Also, to whoever was commenting about watching videos of the battle system and being bored, I have to tell you that that is probably one of my least favorite parts of this game (essentially it's just the KOTOR battle system anyway). What makes WRPGs, and this game in particular, fun for me is conversing with NPCs, following the twists and turns of the storyline, reading the codex entries which provide an amazingly deep backstory for the game, and making decisions and seeing how they play out. If the game were just a bunch of levels with the battle system tacked onto it, I wouldn't buy it either, because combat is really not the reason I buy these sort of games. Some may see it differently though.