«First-person shooter godfather and OpenGL stickler John Carmack has revealed that he now prefers Direct3D to OpenGL, saying that 'inertia' is the main reason why id Software has stuck by the cross-platform 3D graphics API for years. In a recent interview, the co-founder of id Software said, 'I actually think that Direct3D is a rather better API today.' He added, 'Microsoft had the courage to continue making significant incompatible changes to improve the API, while OpenGL has been held back by compatibility concerns. Direct3D handles multi-threading better, and newer versions manage state better.'»
tl;dr Джон Кармак говорит, что, де, у Direct3D API лучше, лучше мультитрединг и вообще, OpenGL мы используем потому как инерция.
Таки дела.