An OpenGL driver capable of running most commercial games is horrifically more complex than a CUDA driver. An OpenGL driver that merely works according to spec is useless in practice. To achieve any practicality for non-trivial use cases an OpenGL driver has to take an attitude of "do what I mean, not what I say", much like web browsers and Windows' backwards compatibility. CUDA doesn't have those problems. NVidia never has to deal with developers complaining that their broken code worked fine on some other vendor's platform. They don't have to worry about programs relying on some esoteric decades-old feature that NVidia doesn't care about but had to implement anyways for standards compliance. And since CUDA is operating in the professional segment of the market, they can take their time when it comes to compatibility with bleeding-edge versions of other OS components.