Believe it or not, there's a larger market on the PS3 for shooters than there is PC. I'm not in any way saying that FPS games are better on the PS3 than they are the PC, but that's where the market is.
Sony, Microsoft, and Nintendo never seem very keen on cross platforming the game servers.
I believe the only thing to ever do this is Portal 2.
I could understand developing on both PS3 and PC and having it on separate servers being a good idea, but never tying the two servers together. Besides, we don't want all the PS3 players getting dominated and feeling bad because they simply refuse to believe that using a mouse to aim is much more precise.