What if part of the reason why later versions of Windows 10 are basically unsupported on older hardware is because Intel deprecated the use of the A20 gate in Haswell. The A20 gate was used to solve the A20 line problem from PC systems using the 80286. And for certain platforms, manipulation of the gate was necessary to access certain peripherals.

So without a guarantee the A20 gate was available, this could've led to unexpected issues for older OSes that tried to manipulate it or hardware that relied on its behavior.