People might just prefer to play it on the XBOX, there's also things like Gamerscore, being able to chat with their LIVE friends etc.

I had a PS3 and an XBOX and got it for the 360 originally, this was purely because our house had a ps3 and 2 360s, one of the 360s and the PS3 were communal and lived in the living room so they could be used by any of our housemates. I had my own 360 from when I still lived with my parents so when I moved out, I kept that one and had it in my room hooked up to my PC monitor. So I got XIII on 360 because it meant I could play it in my room without having to worry that I was stopping other people from using the TV or whatever.

I eventually got it for the PS3 a couple of years later, but that was because I chanced upon a cheap collector's edition in the local game store.