John Carmack historically had a real thing against Direct3D so that's why his stuff has only ever supported OpenGL. Given his plans to port Doom 3 to the Xbox I'd say that stance has softened somewhat in recent times either that, or MS have given him an offer he can't refuse if anyone can afford his price, it'd be MS.
Homeworld did support OGL but it was fairly buggy, at least on my old RivaTNT under Windows 2000 or XP, it used to flicker (not low refresh rate flicker though) so bad I was forced to revert to Direct3D. My new Radeon 9500 Pro doesn't seem to have that problem though, I can use OGL just fine there.