Spam and binaries. Moderated groups moved very slowly as every message needed to be approved. Unmoderated groups became cesspools of spam and flamewars. Binaries groups carried so much data that supporting them was both expensive and legally risky.

As the utility of USENET as a discussion platform diminished due to spam and moderation difficulties, and the binaries groups became an expensive liability, ISPs stopped providing USENET access included with user subscriptions.

That takes us to where we are now. Mastodon is probably the closest successor architecturally, but I'm not sure there's a good example of a platform that has ever seriously solved the problems of moderation in a decentralized environment.

Spam.

USENET if you want to use it is still there, but the amount of spam vs active posts is low.

I'm just getting back into it after a couple years away.

Thunderbird is cross platform and handles USENET nicely.

I use Panix.com to get my feed but there's also SDF.org which lets you post for free once your account is validated (making a donation is a great way to instantly validate your account, I suggest the $36 one-time donation which will give you a full account)

Comes up pretty frequently, you can search HN for 'usenet' for an infinite number of takes. Couple of recent threads

https://news.ycombinator.com/item?id=37336606

https://news.ycombinator.com/item?id=36194941

https://news.ycombinator.com/item?id=31681234

To add to what everyone else has said, there was also new competition from mailing lists administered via the web, and from web based forums.

I feel like Google Groups deserves a (dis)honourable mention here — it was a free web-based interface to usenet (which led some people to think that usenet itself was a Google service), which Google lost interest in, let whither, and eventually shut down. I don’t think it killed usenet, but it didn’t exactly help.

Usenet used to be free with your ISP. Once forum software got popular it sort of took over.