It's way harder to support Linux than Windows from a developer's perspective. Proprietary vs. open source drivers, approach to driver updates (rolling release vs. stable distros), 5 trillion incompatible glibc versions, X11 vs. Wayland etc, janky sound systems with varied support across Linux distributions (Pulse, Alsa, PipeWire), no ABI compatibility guarantee etc.
What has that got to do with Valve providing a compatibility layer so devs can broadly ignore all that nonsense and just target Proton?