It's worth considering what Microsoft's QA costs might be on an individual platform basis. Yes, many earlier platforms (motherboard chipset and CPU combined) can support the security features that are about be implemented in W11, but I'd guess that there's a push-and-pull of cost vs benefit. The inclusion of the Surface makes this apparent.

Then, it's worth considering any features in new platforms that aren't available on the older ones - and any planned point releases that rely on the new-only features. No idea if that's going on.

Also, consider where MS makes money on new OS versions (hint: it's not pre-existing hardware)

Where MSoft is falling down is not waiting until all these decisions are cast in concrete before they announce - but there's the issue of Insiders testing early versions, so they have to say something.

TL;DR: this is typical inter-departmental corporate decision making.