It's not that there's a hard OSX requirement, but Mainstage (for live mixing) dominates this field. It's incredibly low latency, and there's really nothing as good for this specific use case. Combine Mainstage with on-demand sequencers like Doggiebox, and Apple really owns this market.
Yeah, this is the main reason. We use MainStage for all shows. Also, Mac minis are quickly replaceable. If we’re in a city and a computer breaks, we can grab a new computer from the Apple Store right away.
Besides software and familiarity, I'm sure the Mini's form factor has a lot to do with it.
Also, if you need to replace or duplicate a unit with an identical one, you know you can always run out and easily find replacement Mac Minis. If you used some random ultra-SFF PC, can you be sure you can get another identical one easily if you need to? What about 3 years from now? Changing out hardware always introduces a possibility that something might go wrong - exactly what you don't want on a broadway show, a few hours before a performance!
The 2018 Mac mini is, from all reports, a pretty decent machine for DAW usage. Many replaced their 2010 Mac Pro rigs with 2018 Mac minis + Thunderbolt chassis.
I'm a MacOSX plugin developer, and in touch with others also facing this situation.
I develop on a VERY old machine in order to support backward compatibility way way farther back than Apple will allow: my current plugins will run on PPC machines because those can be used as music DAWs. As such, the machine I'm compiling on is not producing 64-bit AUs that will work, directly, on MI Macs. They work on literally everything up to that point, but Apple finally shanked me, at least w.r.t that compile target. Until then I was able to support PPC to present day with one three-target fat binary :)
Another dev, Sean Costello, told me that older builds of his stuff (pre-2017?) weren't running on M1, but everything built past a certain point (a new version of XCode, that had long abandoned things like PPC and possibly 32-bit support) was automatically working on M1 through the Rosetta layer.
So, depending on the build environment, Apple arranged that the audio plugins don't even have to be updated. Depending on the libraries the plugins rely on (a vulnerability for some of the big names that use bespoke but OLD libraries to do things), some of the plugins might need only a recompile to be native to M1 architecture. And some might be really intractable.
