I’m guessing nobody was using it. I think Figma became successful (apart from being a good product) because it was free, so developers try it in their free time, and bring it into their company
macOS virtualization is pricey by Apple's design. This is not just a GH problem
macOS hardware or virtualization is beaucoup bucks in every cloud. As I said, this is not just a GitHub (or Microsoft) problem.
That's just not an option for mac or iOS in the same way it is for Windows.
Besides that, most companies already have CI pipelines in place for all their apps. Why would their switch?