I think the issue right now is that SteamOS is still not stable and usable enough for the average user, and anyone who is a power user probably doesn't want to

I think the issue right now is that SteamOS is still not stable and usable enough for the average user, and anyone who is a power user probably doesn't want to run SteamOS, when they can just run Steam on Linux on whatever distro they want to use. Of course, the other big one is Linux support in games. I have a 600-game library and only a proportion of those are Linux compatible (250-ish, to varying levels of success).