A key aspect is that nowadays we rarely encode pure text - while other encodings are more efficient for encoding pure Mandarin, nowadays a "Mandarin document" m

A key aspect is that nowadays we rarely encode pure text - while other encodings are more efficient for encoding pure Mandarin, nowadays a "Mandarin document" may be an HTML or JSON or XML file where less than half of the characters are from CJK codespace, and the rest come from all the formatting overhead which is in the 7-bit ASCII range, and UTF-8 works great for such combined content.