Because Microsoft chose not to support add-in 3D accelerators on the NT kernel and its successors. Since Microsoft owns Direct3D they get to make the choice. It's very annoying since you can see how easily the cards can be made to run glide and OpenGL games under Windows 2000 and XP. It would have been easy for Microsoft to have allowed such support. But they're... very... annoying.
With out a doubt i have to agree with you on that one. i had one of those cards for a short time and could not get crap to work on it. i was even part of a beta test program for some 3rd party drivers hosted at rage3d, and while the drivers did make some improvements, clever programming could not make up for a crappy card.