NTVDM used to contain an x86 emulator included for running DOS programs under Windows NT on non-x86 platforms (Alpha, MIPS, PowerPC). There is no reason in principle why they couldn't have used that existing code on x86-64. But, I assume they decided that the cost of doing that (in developer time/etc) wasn't worth the benefit. (Running DOS programs has little commercial relevance any more, and the small minority who still have a need for that are better served by more fully-fledged virtualisation solutions such as Hyper-V, VMware or VirtualBox.)