NTVDM requires Virtual 8086 mode in the processor. This doesn't exist in the 64-bit modes, requiring a software emulator. That is
why OTVDM/WineVDM exist.
You can see all of this explained in the README for the very project you linked:
```
How does it work?
=================
I never thought that it would be possible at all, as NTVDM on Win32 uses V86
mode of the CPU for fast code execution which isn't available in x64 long
mode.
However I stumbled upon the leaked Windows NT 4 sourcecode and the guys from
OpenNT not only released the source but also patched it and included all
required build tools so that it can be compiled without installing anything
but their installation package.
The code was a pure goldmine and I was curious how the NTVDM works.
It seems that Microsoft bought the SoftPC solution from Insignia, a company
that specialised in DOS-Emulators for UNIX-Systems. I found out that it also
existed on MIPS, PPC and ALPHA Builds of Windows NT 4 which obviously don't
have a V86 mode available like Intel x86 has. It turned out that Insignia
shipped SoftPC with a complete emulated C-CPU which also got used by Microsoft
for MIPS, PPC and ALPHA-Builds.
```
As to why they didn't continue with that solution, because they didn't want to rely on SoftPC anymore or take on development themselves for a minuscule portion of users who would probably just use 32-bit Windows anyways.