PPC was niche compared to Intel. So they moved from a dying ecosystem to a thriving one.

If they moved from AMD64 to ARM, they would be doing the opposite. AMD64 is not going anywhere on the desktop and is by far the undisputed performance king. Not to mention how many shops and pro end-users they would instantly lose. The number of developers that run macOS and depend on AMD64 virtual machines must be enormous. Given how much Microsoft is embracing Linux, it would be commercial suicide for Apple to abandon the AMD64 platform since they would be driving all these devs to their competition.

That was a different time. They had to do that otherwise they were dead. PPC was not keeping up with Intel.