Partly because Microsoft resisted UTF-8 forever, and so using the ANSI/multibyte strings didn't therefore give you modern functionality. Why they didn't implement Unicode for Win95, I'm just not sure. If they had, the only reason to compile an ANSI version would have been to target Win32S (Windows 3.11).

Or, they could have implemented a UTF-8 code page for Win32 as soon as it was available and then most software could just use byte strings.