Eventually Google's natural language-esque search just got big enough that no other interface to the web really mattered anymore, and their speciality was in parsing unstructured text & messy HTML into simple phrases. Better semantics might've helped other spiders, but by that point nobody cared about other spiders anymore.
For things like social sharing, OpenGraph had the commercial support of Facebook and Twitter and was far simpler to implement. And other communities, like MediaWiki, used RDF only for the low-hanging fruit (Wikidata) while the most valuable info was still locked behind freeform blobs of text (Wikipedia).
The semantic web took more effort to implement than the crap it usually describes. Most humans just don't really want to waste time classifying stuff. The communities that do (science, pirate communities, libraries, etc.) already have their own classification schemes. There was just no need for another web-only classification scheme, no popular desire for it, no sufficient commercial interest behind it, no end-user advantage of using it over HTML... is it surprising that it failed? It tried to solve a problem nobody really had, using a solution that was quickly eclipsed by machine learning. And for the few actors (search engines, social networks) who actually wanted effective classifications, their own algorithms were both more effective and more private, not relying on/enabling their competitors. Open classification excites librarians and archivists, maybe, and nobody else.