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.
As I see it, as long as Apple is putting out x86 hardware, there’s no reason why it can’t be AMD x86 hardware.
(I’m also secretly hoping the ARM thing won’t actually happen, but that’s neither here nor there, and I’m probably wrong.)