Gaming

As others have said, tiny market, but also that it often requires more development for the Linux port to get going, and even more development to actually make it run well. Like for instance, Civilization series usually release with Linux and Mac ports, but those are done by a third-party company which I imagine does add additional costs, and those suck regardless.

Not like it's a bad thing necessarily, the vast majority of native Linux ports I've tried were either severely out of date, had significant performance issues, crashed a lot or had some quirks that would make it not worth playing anyway. It's probably just easier if developers focused on proton compatibility instead.