It's even simpler then that.
It's which games use EAC or BattleEYE and have dog shit devs that won't just turn on proton support. It's seriously just this at this point. I feel like every time I go looking though protonDB a majority of the garbage rated games are anticheat/drm related problems.
Fucking fromsoft games use EAC and they have worked out of the box day 1 on Linux.
I have nearly 1000 games on steam the only ones that don't work on proton with zero fiddling at this point are either EAC/BattleEYE games that don't support proton. Or old games from the early 00s with drm that also don't really work on windows anymore either.
I am also on Manjaro.