I made games primarily for Windows which we also compiled for Linux. It is mostly input/output stuff, aka hardware issues. That is, audio issues, input issues, storage issues, dependency issues. Modern game engine mostly handle the rest. It wasn't such a big deal to fix, but most gamedev lacked experience with Linux, and most projects are already over budget and late, so fixing Linux for an extra 2-5% of sales didn't make much sense at small scale. Proton kind off fixed all of this tho.