maybe because they had limited support for many features?
>Millennium
no textures, alpha blending, transparency, bilinear filtering
>G400
slow but ok, but its predecessor G200 only got OpenGL driver 2 years later with release of G400
Gimmicks that are even today way overused. The world would be better off without them.
Note that I am not into 3d games, and while someday I mean to learn CAD that day is far in the future. If I was into either of those things the features you name would be important. However since I'm not those things are useless gimmicks - YMMV.