https://en.wikipedia.org/wiki/UTF-8#History

That was restricted I believe primarily for compatibility with more limited encodings like UTF-16.

I guess it's possible that at some future point in human history when UTF-16 has been purged from memory, the 5 character encoding might be allowed again. :)

Characters <128 are encoded with a single byte: 0xxxxxxx

Characters >128 are encoded with multiple bytes.

A two-byte character looks like:

110xxxxx 10xxxxxx (11 useful bits, representing code points 128-2047)

A three-byte character looks like:

1110xxxx 10xxxxxx 10xxxxxx (16 useful bits, representing code points 2048-65535)

A four-byte character looks like:

11110xxx 10xxxxxx 10xxxxxx 10xxxxxx (21 useful bits, representing code points 65536-2097151)

Now, technically this scheme could expand to 6-byte characters without getting confused with things like BOM/etc, however any code points larger than 2^21 wouldn't be representable in UTF-16, which has its own set of constraints. This means the unicode consortium has basically limited themselves to two million or so possible code points, which is why UTF-8 doesn't need to go more than 4 bytes. (I wonder if a future unicode version will require a larger limit and would thus create a new "utf8mb6" scheme, and drop UTF-16 altogether?)