It needed a client program.
Digg/slashdot didn't succeed because they were flashier or moderated or had less illegal content or privately developed or any of these other reasons given, it was because people could just follow a link to a web page and they're in. Even today reddit or discord do everything they can to convert web users to app users for obvious reasons, but they must have that web version for casual users or they'll die off to whoever does have one.
So ultimately what killed Usenet was Section 230. Without that services could do simple copying/caching like NNTP (protected under telecom rules) but the actual presentation had to be done on the user's computer. And consequently were 230 repealed it wouldn't be the end of the social media world, but rather the resurgence of Usenet-like models where all the presentation is done by a client program.