Most people want to centralize around something popular, because there's an instinctive desire to be part of the "in" crowd and that is most often defined by what is popular. The convenience that comes with a centralized service is also strongly desired by most people who have bigger fish to fry than keeping track of so many forums and services.
Also, it's not just forums that became centralized; so have other communication mediums. One particularly impressive example is Discord: Remember Internet Relay Chat? AOL Instant Messenger? MSN Messenger? Skype? Yahoo Instant Messenger? ICQ? Teamspeak? Ventrillo? They all became Discord, both in private and professional contexts. Outliers like LINE and Microsoft Teams only survive out of specific cultural or fiscal clout.
>And in that sense, perhaps there is no obligation to understand the entire conversation to participate, you just react to the OP.
HN is also laid out like that, you are also just "reacting to the OP" rather than going down an impractical chain of chronology.
>If single-topic Facebook Groups / sub-reddits exist, then why is the internet "crowded"?
Because it's still <centralized service> rather than <a service>. Some people hate <centralized service> but have no alternatives, unlike 20 years ago when twelve dozen individual services on any given subject matter were just internet'ing away with naught a care if you patronized them or not. You had options, today you don't.
Most people want to centralize around something popular, because there's an instinctive desire to be part of the "in" crowd and that is most often defined by what is popular. The convenience that comes with a centralized service is also strongly desired by most people who have bigger fish to fry than keeping track of so many forums and services.
Also, it's not just forums that became centralized; so have other communication mediums. One particularly impressive example is Discord: Remember Internet Relay Chat? AOL Instant Messenger? MSN Messenger? Skype? Yahoo Instant Messenger? ICQ? Teamspeak? Ventrillo? They all became Discord, both in private and professional contexts. Outliers like LINE and Microsoft Teams only survive out of specific cultural or fiscal clout.
>And in that sense, perhaps there is no obligation to understand the entire conversation to participate, you just react to the OP.
HN is also laid out like that, you are also just "reacting to the OP" rather than going down an impractical chain of chronology.
>If single-topic Facebook Groups / sub-reddits exist, then why is the internet "crowded"?
Because it's still <centralized service> rather than <a service>. Some people hate <centralized service> but have no alternatives, unlike 20 years ago when twelve dozen individual services on any given subject matter were just internet'ing away with naught a care if you patronized them or not. You had options, today you don't.