However, indeed it's pretty thin compared to Windows demos. There's a bunch of reasons, but I believe that there are three big ones:
1. Linux, until fairly recently, required a significant amount of tinkering and tweaking config files before you could even use it decently, let alone code graphics on it. This does not match the average demoscener's mindset; most demosceners want to get stuff on the screen, now. A side-effect of this attitude, by the way, is really shitty code, which in turn has contributed to the small amount of open source demos out there (people are ashamed of the pile of dirt that's under the hood).
2. For most of the nineties and early zeroes, Windows was obviously the strongest platform for 3D graphics. All the graphics hardware just worked, you could choose between OpenGL and DirectX and typically both had decent drivers. You could say that that's unfair because the hardware vendors supported Windows, and Linux drivers were often homecooked by the community, but that's beside the point: Stuff. On the screen. Now.
3. Audience. You could give a Windows demo to your gamer friend and he could see it. Of course, you couldn't give the Windows demo to that classmate who only ran Linux, but you'd prefer not to get into a discussion with him anyway because It'd always turn into a rant about licenses and freedom. Or, well, that's how the common perception of "the average Linux user" was among much of the demoscene.
Nowadays, many active demosceners are game programmers by day, and Windows still is the strongest game dev platform. These people are used to Windows gamedev tools and switching for the evening hobby just sounds too much of a hassle.