It's just not very general purpose right now. It's Arch but the base system is set up as an immutable image. Here meaning any changes to the system are lost on updates. This includes stuff like the Nvidia drivers, so it's my understanding it's of little use for the majority of Steam's install base. The kernel also isn't strictly upstream and last I checked was rather dated with needed bits backported. If your PC however had newer hardware, it could mean you wait very long for support in SteamOS. Also I'm not sure there's a generic installation process. They probably rather have device-specific images.

PC gamers already have plenty of choice with similar options, even HoloISO which is basically SteamOS with needed bits added. I'm not the biggest fan of SteamOS' approach for a desktop system, and I'm not sure Valve would want to support this use case.