From Loot Ninja, Valve Game Designer Tom Leonard had this to say:
And, could Gabe Newell's status as a former employee of Microsoft be another reason?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.