Seems to me it's all about money. They all gotta make it somehow, and if ads are the plan, than it's going to reach this state eventually. User-funded services seem to fare better, but they rarely grow as large. I don't think there's any other easy solution though.
Nonprofits may not be looking for the mega-exit, but they still need to pay the bills somehow, so most of the same forces still apply. They are also more likely to draw partisan ideologues who are naturally more enthusiastic about having a way to put their thumbs on the scales of the discourse despite lower pay and no chance of that lucrative mega-exit.
Government-funded things have too many of their own issues. Practically impossible to start, execute, or pivot fast enough to compete with startups, and subject to political turf wars and overhead. Naturally, nothing much comparable to such sites has sprung out of that.
Nonprofits may not be looking for the mega-exit, but they still need to pay the bills somehow, so most of the same forces still apply. They are also more likely to draw partisan ideologues who are naturally more enthusiastic about having a way to put their thumbs on the scales of the discourse despite lower pay and no chance of that lucrative mega-exit.
Government-funded things have too many of their own issues. Practically impossible to start, execute, or pivot fast enough to compete with startups, and subject to political turf wars and overhead. Naturally, nothing much comparable to such sites has sprung out of that.