Partly because of the 98% of people who run their drivers, none of them use OpenGL. Even on Windows, the OpenGL stability of both AMD and NVIDIA is _terrible_. We shipped a game 4 months ago using OpenGL that literally crashed on every single NVIDIA driver except the very very latest, and we had to spend tons of time ripping out and rearranging bits of the graphics architecture until we found out what was causing the crash and how to get things rendering without triggering it. (Hence the interest of Valve and others in FOSS drivers.)
Originally Posted by curaga
The D3D drivers are far more stable. In this case, the stability has a bit less to do with D3D being a better API and much more to do with D3D actually being used. Recall that Linux users still frequently run non-GL-accelerated desktops, use DDX drivers and EXA/RENDER to get their basic apps on screen. OS X uses an entirely Apple-written driver architecture. Very few Windows apps use OpenGL, and even most of the ones people think use OpenGL actually use D3D: all Windows implementations of WebGL run over ANGLE, many of the "big 3D content apps" have switched over to D3D on Windows, and the games that use OpenGL are almost entirely just simple little 2D indie games that don't do anything even remotely interesting with the GPU.
Granted, Microsoft also has tests for D3D and properly designed the driver model such that every vendor didn't have to reimplement all of D3D internally, while Khronos still doesn't even offer a test suite, much less a core OpenGL framework for the ISVs to build (and even if they did, at this point the ISVs have too much invested in their internal implementations, so a switch is unlikely to happen without some strong-arming).