This is an important point commercially, but it's not just NVIDIA and ATI. You have to think about the whole "ecosystem". The GPU vendors have an interest in driving people to buy the latest and greatest chips. Quality and speed only go so far with consumer applications; at some point you need to add features. Those features are only interesting if games developers can use them to create new titles that people will buy. Thus, the API vendor or standards body needs to work with both communities to develop and share new ideas for hardware features that can be applied as easily as possible into new games.

Of course, the workstation market - where OpenGL is still king - is a different beast. Stability, quality, and performance are critical, in that order, and new features are generally adopted quite slowly.

I'm not an expert on OpenGL and DirectX, so I'll leave it to others to dissect whether the same API can really serve both markets.