Apple: m68k -> PowerPC (32), OS 9 -> OS X, PowerPC (32, 64) -> x86 (32, 64) -> Arm. They've dragged giants like Adobe (kicking and screaming) thru most stages.

Windows NT has always been portable, but didn't provide any serious compat with Windows 4.x until 5.0. At that time, AMD released their 64-bit extension to x86. Intel wanted to build their own, Microsoft went "haha no". By that time they've been dictating the CPU architecture.

I guess at that point there was very little reason to switch. Intel's Core happened; Apple even went to Intel to ask for a CPU for what would become the iPhone - but Intel wasn't interested.

Perhaps I'm oversimplifying, but I think it's complacency. Apple remained agile.