A bunch of shittier smaller isolated networks sounds exactly like what we transitioned away from, you yourself bringing up phpbb. Are you asking why people don't want to register for yet another silo'd community every time they want to talk about a different subject? Or why people may want to talk about their cycling hobby here and there but aren't so fanatical about it that they want to register for bikeforums.com nor mingle with the zealots that post there?
Just sounds like you're romanticizing something that wasn't all that great for most people. A lot of those types of forums still exist, btw. Probably even more than ever. I run one. My forum fills a niche that doesn't translate well to a place like reddit (longform collaborative writing). These communities are still here for the people looking for them, but it's no mystery to me why they have limited appeal.
Frankly, the modern web has enabled more people to participate that aren't as nerdy or fanatic about a niche nor willing to jump through hoops like joining a new forum with its own rules and nobody they know irl. Not everything is going to shit, there are simply always trade-offs.