The semantic web failed in large part because browsers didn't implement XSL, and so the average web developer never learned about the idea of sending structured data to the client as XML and using XSLT to instruct browsers to render the data as XHTML. If these specs hadn't been released in the middle of Internet-Explorer-monopoly-induced-browser-stagnation then they wouldn't have been dismissed as failures by the time browser competition increased and standards started to be honored.

I'm not sure that the semantic web would succeed even if it were a new idea today. HTML5 was popular because it gave web developers tons of shiny toys and got them excited to use it. XHTML was conceptually beautiful but it hardly enabled any user-visible improvements over HTML4, so only the philosophically-minded web developers gave a fuck.