There are lots of reasons I use Direct3D as opposed to OpenGL. If you look at the chart on this page:
http://www.xmission.com/~legalize/d3d-vs-opengl.html, you'll see an awful lot of "No's" in the OpenGL column where thare are "Yesses" in the DX8 column. But the real answer is that I speak Direct3D and not OpenGL. If I was a writer who had written a best-seller in English, would it be logical for me to abandon the language and try to write the next one in French, if I don't currently speak that language?
Not that I'm really all that thrilled about Direct3D. It's like the old saying about Democracy -- it's the worst possible form of govenment, except for all the rest.