They're pretty upfront about the reason - their anticheat supports Linux, but enabling it would make it much easier to cheat because it's not nearly as effective on there, and they decided the cons outweigh the pros.

Apex Legends went through the same issue when they enabled Linux support, cheaters swarmed to Linux en-masse because it was so trivial to evade detection even with free/public cheats, and after a year or so the devs threw in the towel and blocked Linux again.

They're not doing this out of spite, they'd be happy to take your money if there were no downside, but unfortunately it is a trade-off for games which are sensitive to being ruined by cheaters. At least for now.

> The game runs fine on Linux, the devs just don't allow it.

The native Linux build never worked that well. Something was always broken because Unity's Linux support is/was spotty. Upgrading Unity versions would break random things.

Anticheat is the issue holding back Proton support, though.