i'd always assumed its because direct3d has no quad buffered stereo and no swap groups/genlocking. The entire Quadro line of cards would be impossible if it weren't for OpenGL. As I've said before, if d3d had these features I wouldn't be here having this conversation now. I'd be using d3d with all of its tool chain.
There's also the control aspect - they need an alternative to MS which they have control over.
None of this has anything to do with linux - that's just wishful thinking.
Having said that, because it seems to be impossible for these features to be implemented in Vista (and obviously OSX), linux may become the only option for visualisation in the next few years. That's unless it's impossible in X then, what with all this compiz fusion crap.