I don't really believe this to be an issue - Valve directly contracts CodeWeavers, they developed Proton together, and they've been pretty clear from recent hiring bursts that it was specifically to work on Proton. I have to imagine the income from Valve is exponentially higher than the relative niche of CrossOver. They're basically a subdivision of Valve now.

I'm interested in alternate explanations if you have them, to be clear this is only my theory.

While Valve might make up a big part of CodeWeavers book of business now, that was not true when the original contract was signed.

The alternate explanation is that it's rather pointless to integrate it because all new Macs are ARM, and there are basically zero Windows games compiled for ARM.

Not likely considering Valve are working on an ARM version of proton in the open, have published test results for x86-64 windows games playing on proton-arm64ec-4, and there are credible leaks that the Steam Frame uses an ARM cpu.

If anything Valve is moving towards ARM and taking the library along with it.

Then it will probably happen after they get it polished on Windows.

Crossover isn't aimed at the gaming crowd, but productivity software. Their builds favor stability over new and shiny compatibility/feature support. I just had a license because I like supporting CodeWeavers.

They also make a Linux build of CrossOver.

They advertise gaming as a use case for CrossOver (there used to be a separate CrossOver Games product, even, which I was a customer of), and it would not surprise me if the linux build of CrossOver sells so little they wouldn't care.