Personally, I think this is related to the differences in approach to developer relations between Microsoft and Sony ...
Both the PS2 and the PS3 launched with inadequate tools and documentation and (essentially) their response to developers’ complaints was “Tough!” ... In contrast, with the XBox and XBox 360 Microsoft launched with an excellent toolset and gave developers access to experts to aid developers.
While I know nothing about the inner workings of Valve, I wouldn’t be that surprised if some developers have started to resent Sony in part because of how they were treated in the past, and because of how much better Microsoft has been to them lately.