Speaking to Loot Ninja, Valve's Game designer Tom Leonard explained the reason why Left 4 Dead and its sequel are only available on PC and Xbox 360:
The PC and the 360 are just more straightforward. 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.
This is not the first time that Valve criticizes the PlayStation 3. Valve co-founder Gabe Newell has trashed Sony's PlayStation 3 several times, explaining that the complex architecture of the platform makes the development of games for that system a "waste of everybody’s time."
http://news.teamxbox.com/xbox/19936/...Launch-on-PS3/