https://en.wikipedia.org/wiki/Cancelbot
Many Usenet users felt that there doesn't need to be a worldwide consensus about this (it's OK if different sites have different policies or different views of the contents of a newsgroup). However, the deployed attempts at antispam and content moderation through cancelbots and counter-cancelbots didn't scale very well or to many users' satisfaction.
I also think that moderated groups had a challenge with authenticating moderation (to prevent people from spoofing approvals).
Another scaling challenge, I think, is that Usenet originally had the model that there would be one single worldwide discussion forum about each topic. In later systems people often wanted friend-group-based, geographically-based, or project-based forums as well (although Usenet did end up having some of these, like organizationally-scoped news hierarchies!). It's daunting to imagine being on the one single worldwide talk.politics today.
But the reason it isn't very popular, I suspect, is because it is 99% spam. Only the occasional post is actually content. And most of the newsgroups that were popular once are only spam now. You can tell it's been going on a long time because there is reams and reams of it, with nobody even bothering to reply - literally spamming the void. It's really a disaster.
Back in the 90's people saw that coming. It's definitely here.
In the early 90's I used to provide UUCP news and email to BBS systems. If NNTP content was as crappy then as it is today, I would never have bothered.
But Google Groups is probably the only thing that could handle a large scale influx of users.
There simply aren't that many free Usenet servers.
If Usenet were to suddenly explode in popularity for some reason, those users not wanting to put up with Google Groups would probably overwhelm the small number of free NNTP servers.
Speaking of servers, what (almost) killed Usenet was ISP's shutting down their NNTP servers. It used to be that every major ISP ran an NNTP server you could connect to, similarly to how they give you an e-mail server. This service was obviously not free; it was part of the service package, though, so it looked free.
A big reason they stopped doing that was that most of the NNTP volume came to consist of copyright-infringing binaries.
People are now used to actually free discussion forums like Reddit; any resurgence of Usenet will have to use free servers (and your ISP won't provide a free-looking one any more). Too few users are willing to pay for Usenet access; a Usenet resurgence based on paying users connecting to servers cannot happen.
There is little economic incentive for anyone to provide a free Usenet server, because there is no revenue. You cannot collect any accurate personal information, and you cannot serve advertising. I suppose NNTP could be hacked to insert advertising into articles. These ads would likely be ineffective in their limited format, and face a backlash. Still, that's an idea. For instance, the non-paying users of an NNTP server might not get to see the original signatures of articles; the server would replace it with its own, containing an ad. Moreover, when posting, non-paying users would have their outgoing signature replaced by an ad. Paying users would see original signatures and get to send their own.
Before BitTorrent, people posted binaries as multi-part MIME or uuencode jobs to Usenet, believe it or not, using multiple articles. The clients still support automatic decoding of binaries from multiple posts.
It would be nice for Usenet to come back without the alt.binaries.* cruft that is not so useful today anyway.
Sure it's technically scalable, but it's pretty noisy even now. Without some revolution in client design I suspect most groups would roil and gibber uselessly like the surface of an imageboard, if subjected to hyperscale-y traffic.
It can't, and doesn't want to be, monetized, therefore big players hate it. Just look at how Google first destroyed DejaNews then removed the discussion filter from the search engine.
Also Usenet comes from when the Internet was a place for gentlemen, therefore is very open to abuse. If it was extended with basic security in mind and allowed very small binary support in line it would become a killer protocol that could displace many proprietary ones.
Hahahahahahahahaha
It’s a complete wasteland of meaningful discussion, everything is spam or pirate porn or copy-pasted nonsense. I have no idea what you would legitimately use it for now.
https://trends.google.com/trends/explore?date=all&q=%2Fm%2F0...
In the end, Usenet really taxed the resources of an organization and I think many sys admins just decided all the time and energy required to keep the server up was not worth it.
[0] https://en.wikipedia.org/wiki/Usenet#Usenet_traffic_changes
There are still good discussions going on in particular newsgroups. You have to do a bit of searching. comp.misc would be of interest to Hacker News readers.