Valve has admitted it doesn't develop for PlayStation3 because Xbox 360 is just more straightforward.
"The PC and the 360 are just more straightforward," Valve design Tom Leonard said. "We can focus on what we want to do, which is make game experiences, instead of sweating bullets over obscure architectural decisions they make with their platform."
"I didn't come into this business in the 90s because of some technical fetish. I came in because I wanted to give people experiences that made them have fun."
Ouch. So if you want to know why Left 4 Dead 2 is a console exclusive for Xbox 360, there's your answer.