Simple, Crysis 2 was designed for the PC.
The Xbox and the PC have very similar programming codes. (the ps3 uses a different type of programming)
Therefore, it is easier to take the awsome graphics on the PC and turn them into great Xbox graphics, than it is to change it into good Ps3 graphics.
it all comes down to us getting shafted by lazy devs. (the game is still fun though)