There's actually a very specific reason for it to fail. There's an unfixable bug in the way that the Windows emulates 16-bit BIOS calls in the 8086 mode monitor

There's actually a very specific reason for it to fail. There's an unfixable bug in the way that the Windows emulates 16-bit BIOS calls in the 8086 mode monitor. This is specific to Microsoft's implementation and is (if I remember correctly) an architectural issue (and not the only one). The options are rewrite an emulator or ditch 16-bit support in 64-bit. I'm not saying that this is the only reason, but it is one. Emulation/Virtualisation is different, but gets the support requirement off Microsoft's books.