Most of them had quarter/midlife crises and got partners/children/mortgages etc. and no longer care. The next generation know nothing else other than the current internet.
Yeah sadly that's it. The great forums all died off when it was no longer doing ridiculous stuff to entertain internet friends, and instead having super lame adult responsibilities.
Speaking for myself, kids even with a hands off parenting approach are a ridiculous amount of work.
HN is one of the closest to it IMO. I sometimes think the very Web1.0-ness of its interface and its hostility to mobile browsers are purposefully made speed bumps before becoming fully mainstream.
Also time passes and people change. Many of the people on HN today were once dialup users. They are the same people but also different.