>>10
UTF-16 is a variable-length encoding, and hence not much simpler than UTF-8
It is very much simpler handling either 2 or 4-byte characters only, compared to 1, 2, 3, or 4 for UTF-8.
As for the second: consider HTML
HTML is not the only form of text there is.
The encoding is inflexible
In 20+ years only ~10% of the whole code space has been assigned. 1.1M characters is plenty already.
Ugly encoding
Subjective. As if UTF-8's multiple shifts and compares are any better?
Here is someone who has written UTF-8 and UTF-16 codecs, so I have first-hand experience that UTF-8 encoding is more complex than UTF-16.