The Internet has simply grown too large over the past couple of decades for any unmoderated public space to not be taken over by people who don't care about community norms, individual bad actors, organized invasions, and psyops.
Usenet could handle the first two in small doses: people who don't care about community norms will eventually learn or leave, bad actors will get bored of trolling, and persistent individuals can be killfiled. But both of those two flooding in in large groups can kill a community. If bad actors harass and attack every new person every time they post something, the community can't grow. and enough people in a community who disregard the existing norms will simply cause the Overton Window to shift, establishing a new normal. And a killfile isn't a large-scale solution: when you have to have a triple-digit killfile just to get past the noise and actually see the useful discussion, the community is dead.
Usenet on the other hand has never been resilient against coordinated activity. The Meow Wars were one of the most deleterious things to ever happen to Usenet back in the day, and I'm sure it contributed a lot to people abandoning Usenet for moderated web forums.
Now that the Internet has had 20 more years to further develop invasion techniques, and the invading forces are even larger and more malignant than ever, unmoderated communities can't survive unless they're invisible. Even heavily-moderated communities have trouble handling incoming raids from 4chan and 8chan.
And then you have the subtle psyops, groups stealthily infiltrating others in order to promote an agenda. Imagine a coordinated effort to have new people join a newsgroup for a TV show and slowly push the Overton Window towards normalizing antisemitism. This is hard to detect and root out even in a place with moderators (see: Stormfront's psyop in /r/videos), and unmoderated spaces are completely helpless against this kind of assault.
