The comment talked a bit about why they chose to stick with OpenGL, but mentions only "inertia". I suspect "we can support Linux and Mac, maybe some mobile platforms too" might also have a bit to do with it.
It's a bit philosophical too. D3D is an abstraction of graphics hardware with enough high-level constructs to make it useful to real programmers. OGL is an abstraction of 3D principles, with enough low-level constructs to make it perform on real hardware. You can't "unplumb" that overnight; it's how you think.
But presumably it is at least part of the reason id have stuck with OpenGL and not switched to the 'better' API