I’d like to know too! I’ve been told that in general Valve isn’t easy to get a hold of, and that it has contributed to some handheld makers’ decisions to launch on Windows.

But... Valve has clearly been working towards a SteamOS that works on more handhelds, as you can see if you launch the desktop Big Picture Mode on Windows handhelds. Also, Valve is a tiny company of roughly 300 employees.