Maybe because they require hardware manufacturers to use a known set of processors (see https://msdn.microsoft.com/en-us/library/windows/hardware/dn...)

Getting Windows go boot on the completely fictuous qemu -M virt platform for both arm and arm64 was very easy, USB and SCSI worked after less than a week of fixing Qemu emulation bugs (GIC).

You're basically patching the hardware to work with immutable software (as you won't have the windows sources). At the same time, the QEMU ARM virtual system and the Microsoft designated ARM SoCs are perhaps closely compatible whereas it would be very different with other ARM implementations.

Only GIC (interrupt controller) bugs needed to be fixed. The GIC is standard since the Cortex A9. On the A9, you have to use an HAL extension (declared in ACPI) for the timer(a sample is available on MS' Git repo). On the A7/A15 and later, Windows uses the GTDT ACPI table only, so nothing is needed :)

Windows on ARM uses UEFI and ACPI, it can even boot with zero drivers on Cortex A15-class(architectural timer) and later HW. Of course, you only have USB and eMMC in such a config, the driver model is also stable.

Microsoft care. Google don't.