Because it doesn’t work reliably on the surface pro 4. Yes, I have tried surface-linux, and no it doesn’t work well enough. When shutting down, the machine actually doesn’t shut down and my battery was dead the next morning. The boot process sometimes hangs. The OS doesn’t properly differentiate between finger and stylus. It doesn’t seem to do palm recognition. Etc…

I know this is a special case: hardware with specific Microsoft firmware. But I imagine that other people have other specific cases.