The model of QA that Stack Overflow and its various forks follow the same approach struggle with the 90/9/1 problem (
https://en.wikipedia.org/wiki/1%25_rule ).
Q&A was designed to handle the social explosion problem and the eternal September problems by having a larger percent of the username take an interest in the community over time and continue to maintain that ideal. Things like comments and discussions being difficult is part of the design to make it so that you don't get protracted discussions that in turn needs more moderation resources.
The fraction of the people doing the curation and moderation of the site overall has dropped. The reasons for that drop are manyfold. I believe that much of it falls squarely upon Stack Overflow corporate without considering second order effects of engaging and managing the community of people who are interested in the success of the site as they envision.
Ultimately, Stack Overflow has become too successful and the people looking to it now have a different vision for what it should be that comes into conflict with both the design of the site and the vision of the core group.
While Stack Overflow can thrive with a smaller number of people asking "good" (yes, very subjective) questions it has difficulty when it strays into questions that need discussion (which its design comes into conflict with) or too many questions for the committed core group to maintain. Smaller sites can (and do) have a larger fraction of the user base committed to the goals of the site and in turn are able to provide more individual guidance - while Stack Overflow has long gone past that point.
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Stack Overflow and its Q&A format that has been often copied works for certain sized user bases. It needs enough people to keep it interesting, but it fails to scale when too many people participate who have a different idea of what questions should be there.
There is a lack of moderation tools for the core user base to be able to manage it at scale (you will note the history of Stack Overflow has been removing and restricting moderation tools until it gets "too" bad - see also removal of 20k users helping with flag handling and the continued rescoping of close reasons).
Until someone comes up with a fundamentally different approach that is able to handle moderation at scale or sufficient barriers for new accounts (to handle the Eternal September problem), we are going to continue to see Stack Overflow clones spout and die on the vine along with a continued balkanization of knowledge in smaller areas that are able handle vision and moderation at a smaller scale.
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Every attempt at a site I've seen since (and I include things like Lemmy in this which did a "copy reddit" and then worry (or not) about moderation) have started from a "get popular, then work on the moderation problem" which is ultimately too late to really solve the problem. The tools for moderation need to be baked into the design from the start.