Earlier quoted context omitted.
Because Microsoft, much like most of the ecosystems around ARM, didn't build their own CPU. They got an off the shelf mobile chipset, customised it slightly, rebranded it and then chucked the whole shit heap of windows on top.
I have also heard that Microsoft put lower priority on the x86 emulation layer, because they felt like if it was really good, developers would have no incentive to make native compiles. While this sounds instinctively ridiculous to me, as of late I’ve become very cognizant of general laziness in our industry, so who knows…
It's not surprising to me, Microsoft has always prided on having backwards compatibility as much as possible so this is in character for them