Keith

Oriental names do normally render the single-syllable family name before the bisyllabic personal name:-

eg Chiang Kai-Shek was, in our terms, President Chiang; Mao Ze-Dong was Chairman Mao, &c.

I learned this while lecturing at universities in China for the British Council on English Folksong a few months before Tienanmen Square.

So Ban Ki-moon is indeed Mr Ban, as Dave says; and that is, as you ask, why.

They themselves are the ones who say so, so I think we should accept & respect it.

Regards
≈M≈