1) Valve is pissed off Windows 8 because of its store
2) Valve want to promote their own system. Happily Linux based.
3) Valve wants as many users as possible
3a) As many SteamOS users as possible
3b) With LOWER priority they want sell "more" games. Their shop is running great already.
4) A BigPicutre Mode closed system (at least for normal users) is not attractive and not good for point 3) including a) and b)