[jerf]

I think they were trying to keep Unicode fitting into 16 bits. We later gave that up, but now it's kinda too late to go back and de-unify.

[dhosek]

IIRC, ISO-10646 initially preserved each of the 16-bit national codings for Han characters (including three separate Chinese encodings). I don't know if that has been retained at all in later versions of the standard and a cursory reading of the relevant wikipedia pages is not informative.