Its about practicality.
Even big studios cant handle supporting a native linux version and having it run well while supporting windows versions.. which usually ends up with people running the windows version via proton over the native version (Looking at you, Cities Skylines, Truck Simulator games, and more)
So I would say take the easy route and develop for windows with checks to make sure it runs okay in proton, probably a lot less headache and effort than trying to create a seperate native linux codebase.