I think SteamOS was mostly a political play, at the time microsoft looked like it was turning windows into a walled garden platform.
Additionally the OUYA had some mind share and Valve was pushing Steam Machines and their Steam link so a non windows OS to run on them made sense.
On reflection perhaps the Steam Machines and link were a response to the Windows Store as well but I cant remember the timelines that well.
Good news is that didn't happen and Valve continued to improve gaming on Linux for everyone for free anyways.