Ghosteses seem to underestimate the amount of people who owned a 360 but no PS3. Few people would buy a PS3 to play FF13 when they already had a 360. If they did, that's what would be a crazy thing to do, not buying the game for 360.

If you have both consoles, you might get it for 360 if you care about gamerscore more than having the best graphics. If they already have a lot of achievements on their 360 account, they might just want to stick to using that.