I think the linux ecosystem simply doesn't lend itself to gaming very well.
The good thing about Windows is that everyone has the same Windows: the one from Microsoft. Which also allows Microsoft to have full control over DirectX.
With linux, you have all sorts of parties making all sorts of variations of the same thing. So you never know what kernel version you have exactly, what patches may or may not be applied to it, or what implementation of OpenGL you have, to name but a few things.
I think for that reason, something like SteamOS may fall apart eventually. Valve can make their own distribution, but they do not control all the code they use. So if new versions of the kernel, XFree86 or any other parts cause problems, Valve will either have to stick to older versions indefinitely, or create their own patches/forks, and maintain those indefinitely.
Because it will be unacceptable if new versions of SteamOS can no longer play existing games, that people paid good money for on Steam.
It's a house of cards.
Your comments on Linux varieties are very true. But, how about a certain flavor of Linux just for gaming. I think that would be very cool. One of the big game developers could go to AMD and Nvidia and possibly get something going. Imagine, a dedicated OS just for gaming. I won't hold my breath though.