Allowing more than one lead byte is a complication of the existing standard. As many others have pointed out, we have plenty of coding space without that (e.g.

Allowing more than one lead byte is a complication of the existing standard. As many others have pointed out, we have plenty of coding space without that (e.g. by allowing 5- and 6-byte UTF-8 again), so the case for the extra complexity is currently not compelling.