You need to know your history in order to understand this. Very briefly: Development on Windows NT began in 1989, three years before UTF-8 was even invented; a

You need to know your history in order to understand this. Very briefly: Development on Windows NT began in 1989, three years before UTF-8 was even invented; and there was a difference of opinion between the ISO people and the Unicode Consortium over Unicode being 16-bit or 32-bit, with Joseph D. Becker famously publishing a paper in 1988 declaring that 16 bits were sufficient for Unicode ("with a safety factor of about four").