From memeory - it's the lack of a working Linux plugin for Unity's XR plugin framework. We used to use SteamVR on Linux but that's not currently working with the new framework and we need to move to that to support a wide range of hardware.
I'm sure it will get fixed - if not by Valve then by Monado who are doing great stuff for open source XR support.
Sadly, .NET/Mono apps not so good for Linux.