There are publishers who do not offer Linux native versions of games available elsewhere, because GOG does not offer multiplayer and achievement support (ergo Galaxy support) on Linux. That is the only thing which can be faulted to GOG really.

These publishers had no problem providing Linux native builds without achievements and/or multiplayer support through GOG when Galaxy was not a thing yet. At the time they knew they would have to provide a real DRM-free build for GOG, and if they were OK with doing it for Windows they were usually OK with shipping it for Linux too.

Yes, but since then the... other store effect took over, and now some publishers want feature parity between OSes like that... other store provides, or they bail.

The Steam hegemony was already a thing long before that ;)

The real change was the introduction of GOG Galaxy for Windows and MacOS. That’s when Linux native builds started to become Steam exclusives.