Because strategically, the move to ARM makes more sense for them to focus on even if sticking with Intel is a bit more painful in the short term. They already have a large team working on ARM processors and an architecture license for the platform. With x86, they are basically just resellers. So adding AMD's flavor of x86 to their lineup would likely be seen as a distraction for them without providing a long-term benefit.
Primarily, compatibility with legacy apps, and compatibility with other OS's (eg Bootcamp/Parallels).
I also have major concerns about raw performance at the high end, and I suspect ARM would come with even more software lockdown, although there's no reason that has to be the case.