I feel that a lot of hardcore Metal Gear Solid fans, bought there Playstation 3, thinking that Metal Gear Solid 4 would be on their system. It'd be a stab in the back, if they paid $500+ and found out, that 6 months/year later, the game is released on a cheaper console.
Also, have you seen the amount of product placement from Apple & Sony?
They'd have to edit that out, compress the hell out of the game, which would still make it multiple discs. Also, get rid of the 360 jokes.
Getting a team to do that and the porting would cost them more then benefit them.
You see:
XboX360 is hot, very hot in the US. But everywhere else, it doesn't hold as strong of a position as it does there
So, technically it would bring the userbase to about 10.08 million .
Smaller then the PS3 userbase.