I've dabbled in 3D programming, I haven't done anything amazing, let alone made my own game, but I will say that D3D is much easier to write working code than OpenGL. With OpenGL I got about as far as a spinning triangle on the screen. With D3D I was able to get to the point where I was rendering a model on the screen and manipulating it with pixel and vertex shaders.
Somebody earlier hit the the nail on the head when they said that because it's easier it's what people will start and subsequently stick with. Not only that, but as a business if it takes your programmers 10 months to write a graphics engine in D3D and 12 months to write one in OpenGL, which one are you going to go for? There aren't many instances where I will go to bat for MS, but DirectX has a better graphics API compared to OpenGL.