I presume I'm not alone in immediately looking at the source view and wondering "How did HTML win as the document markup format? Wasn't there any effective competition?"
HTML and eventually the web came out of CERN and HTML was based on existing SGML tools in use there for sharing research documents. There was no "effective competition" because there was no market in which to compete.
HTML isn't beautiful but it's simple and effective enough that people were able to write their own websites in the old days. What alternative would you have suggested?
Certainly not RTF... like you said, there didn't seem to be an alternative at the time.