Microsoft must have hate OpenGL otherwise they wouldn't give it so poor support in NVIDIA cards
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It's about driving out business competition.
As to why they didn't go away with OpenGL completely I guess is to give pretense that NVIDIA cards support OpenGL and to prevent backlash from gamers or to mitigate it.
I'm sure in the future they'l go away with OpenGL completely even for now it's already almost unplayable.