And AMD can't handle OpenGL. I don't know why, I'm not sure what's so hard, I'm not sure if there's a monster that guards the OpenGL specs in the AMD office or something, but they have sucked at GL for over a decade, and show no signs of getting any better. They can't claim it is because of an API limitation either. For whatever you want to say about the mess that is OpenGL, nVidia makes their GL drivers dead even with their DX drivers. You can use either rendering path and can't tell the difference in features or speed.
That is also why I'm real skeptical that Vulkan is going to do anything for AMD. While they are heavily involved in the development, they are involved with OpenGL's development too (ATi was a voting member on the ARB and is a promoter with Khronos Group). Given that Vulkan is heavily GL based, originally being named glNext, I worry that AMD will suck at performance with it as well.