I can discover interesting subreddits by pure chance by following a google search on a question I have. Or following a link on a blog
There’s no chance of that happening on Usenet. For most users, search/discovery is a web thing (web search, links) and there’s no link from the web to usenet. Therefore if you don't know anything interesting on usenet you can't easily discover it. Any interesting post/discussion cannot be simply linked on a tweet (or, for that matters, on IRC). Usenet is completely insular. A web forum is not.
Were I a Usenet enthusiast, that would be my 1st priority. Public archives of most important hierarchies (comp.* for example), indexed on Google, easily linkable.