This is my slightly-conspiracy guess: Apple has oodles of old hardware lying around that they would like to keep using but is either too old for macOS or they want to use it for backend services (prod or non-prod, doesn't matter). Think capital expense budget. So if Apple can run Linux on all that hardware, that's a lot of computing power still available for years to come. And if you can get the OSS community to do it for you for free - even better!
I would struggle to believe that Apple had an easier time getting someone else to support Linux on their hardware, than just... supporting their own hardware with their own OS with their own drivers that they already have using internal developers who already know how to write Darwin code for Apple hardware.
Machines too old to run that latest version of macOS can probably be replaced with commodity hardware in the cloud for a few dollars a month. Doesn’t seem worth bring a bunch of decade-old hardware online for this.
