I think some publishers are reluctant because on Windows, those anti-cheat systems involve out-and-out rootkits, so allowing Proton support for them is seen as

I think some publishers are reluctant because on Windows, those anti-cheat systems involve out-and-out rootkits, so allowing Proton support for them is seen as a relaxation of their 'security'— the Linux ports of those systems are userspace only, Proton and Wine don't have that kind of kernel access anyway, etc. I think if, over time, it looks like those fears are unfounded and there are few practical problems with anti-cheat for competitive games on Linux, then probably some currently-hesitant publishers will relent and enable their games on Linux. But only time will tell, I suppose.