[gpvos]

Also, UTF-16 was only published in July 1996 (although the need for more than 16 bits was probably apparent a bit earlier). So before that, Unicode was only a 16-bit encoding, and UCS-2 was enough. UTF-8 was initially just a nice trick to keep using ASCII characters for things like directory separators (/) and single-byte NUL terminators. By 1995 its superiority certainly wasn't apparent yet.

Also, Windows internals were completely 16-bit-character based, including e.g. the NTFS disk format, so by 1992 that was already quite hard to change.

That said, it is crazy that NT didn't have full UTF-8 support, including in console windows, by about 2000.