Nvidia works with a lot of studios to do that stuff. ATI really doesn't, at least not as much. Nvidia having a higher market share than ATI probably helps a bit too.
I suggest you read this http://www.bit-tech.net/bits/interviews/2010/01/06/interview-amd-on-game-development-and-dx11/1
its a great interview with ATI about their developer relations, from what i gather they have just as much "clout", the only difference is Nvidia markets their involvement more than ATI/AMD does. really you can tell just by this thread alone. Just because its a TWIWMTBPed game doesn't mean ATI didn't also have any sort of developer / engineer relation with that game. its just more that Nvidia likes to slap their name all over everything and because of there is a sense that Nvidia does and ATI doesn't.