A push towards more standardised 3D APIs is why.
I'm in two minds about this - On the one hand, I liked Glide; It was heavily based on a subset of OpenGL, and a really nice API to code with.
In fact, when learning 3D programming basics, Glide is a great API to start off with.
Of course, it was 3Dfx-only, and thus propriatory which is not a good thing. I like Standards.
That said, the main Standard we're stuck with is D3D, which is *not* a nice API to code with IMHO - Everything seems done in really odd ways. I don't know if that's just because I was used to OpenGL and Glide, but I really hated Direct3D.
(This may have changed quite a lot - Most of my D3D experience was from DirectX3 to 5...!)
OpenGL will never compete with D3D I think - OpenGL is a stable API which was primarily create for use with CAD, 3D Modelling etc.
D3D is designed purely for games, so the standard is not very well thought out and full of holes, but it's on the cutting edge of graphical effects.
OpenGL extensions gave OGL an edge in the early days (Voodoo2 owners loved OpenGL because D3D didn't support multi-textures for a relatively long time!), but now D3D is evolving a lot faster; In fact, it's driving the feature-set of modern 3D cards instead of the other way around as it was when OpenGL was the bigger API!
I just wish DirectX didn't break backwards compatibility all the time - Some of my more obscure older games blow up with DirectX 9, but luckily I can still dual-boot to Win98, and even use Glide if I need to (Yup, that Voodoo2 SLI is still hooked up

Now if only it bloody worked in Win2k!!)