I chose DirectX mainly because it is simpler then OpenGL with all extensions.
Granted I have not looked too deeply into DirectX because I hate the idea of being locked into any one specific platform (especially Windows!), but from what I see it's pretty similar in terms of level of difficulty! Could it be, maybe, that DirectX is just what you know and have chosen to stick with it? Not that that's wrong!
I also think that Linux needs to mature more on the 3d front before it is really usable (for normal people). For example I remember how I searched the web for a month before I found a patch to make the ATI driver to work with my agp chipset.
Then I must be a bloody genius because I am writing a LWJGL based application than runs under Linux for work!
But seriously, if platform dependance is not a big deal for you then cool, but I fear that you have severely limited the number of people who are going to use your library!