Eh, PPC doesn't run 680x0 natively, does it? Doesn't MorphOS and AmigaOS4 have an emulation layer for this? Why can't AROS?
MorphOS and OS4 only emulate the CPU, but nothing else. They are otherwise API compatible with AmigaOS 3 and old software run under PPC native OS without recompiling. Old programs use PPC native libraries, GUI toolkits, etc, which makes them really fast comparing to UAE environments that emulate the whole Amiga machines/OS/components. This kind of solution is only possible efficiently because PPC and 68k have the same endianness, and that's not the case with x86/x64.
On AROS (x86) you have to use UAE based solution for classic software. It is slower because it emulates the whole machine, and what is worst in my opinion, is that 68k programs run isolated from the main OS. You can't use 68k libraries, device drivers, shell commands, etc under AROS like you can do with MorphOS/OS4. Programs can't interact with ARexx etc.
On AROS you have to re-compile everything you want to run under AROS itself, and I think that's the main point why AROS had so slow start despite the lead of years, because there just weren't source codes available for all you need. On MorphOS and OS4 we had to use a lot of 68k binaries to fill the gaps on native software selection. Nowadays less, but I still use pretty much 68k stuff in my everyday system. But without this kind of transparent integrated compatibility MorphOS and OS4 wouldn't have got anywhere ever, and it's still what appeals many people and makes these operating systems feel like real continuity to classic Amiga, and not being just yet another UAE boxes.
