Apple's migrations were different
PPC CPUs were faster than the available M68K CPUs, and x86 CPUs were faster than the available PPC CPUs.
With ARM they go the wrong direction. What would be the incentive for people to "upgrade" to ARM Windows on an ARM PC for less performance? People would keep buying x86 CPUs and stick with an x86 version of Windows.
Its like when Intel hoped to force the PC market to Itanium to get 64 bits. They thought by withholding 64 bit CPUs people who needed 64 bits would go Itanium, and as memory sizes increased eventually even desktop PCs would go Itanium (i.e. the last dregs of x86 CPUs would be sold in low end PCs today) But they didn't figure on AMD successfully creating their own 64 bit extension to x86, and Microsoft accepting it.
This is the one flaw in the idea about Apple transitioning the Mac to ARM. Yes, Apple's SoCs are by far the fastest ARMs around, and compare favorably on a performance per watt basis with Intel's x86 CPUs. But they are still only half the performance of Intel's highest end x86, and while a design targeted at using more power could bump that up somewhat, it would still mean a transition that costs performance - a hard sell for Apple's professional creative market where a performance drop of 5-10% might be tolerated, but not 30-40%.
Now there's no reason to think that it is impossible for Apple to design an ARM SoC that matches Intel's performance, and if Apple can do it Microsoft could (eventually, once they had a competent team like Apple does) do it. But it hasn't been done yet, and simply matching performance just gives you a migration for migration's sake. Apple could get away with it (especially if it meant bringing Continuum-like capability to the iPhone that could really run all Mac apps) but Microsoft has almost no userbase to amortize those development costs with - though I guess they have proven over the past 15 years that they're not averse to throwing billions down a black hole so who knows?