The knack and knowledge of hosting a dedicated server required skills. If you could one click button host a Quake 3 Arena server or UT, there would still be sort of a crowd.
But not only that, if you were new: servers got toxic with the same grumpy players knowing every single trick in the map to one-shot kill you with a rail gun. It got frustrating playing the same bloody map each map change. "Oh yay, it's cs_office again" /s
The whole genre got stale and never has been successfully reignited. As Discord to TeamSpeak; same misfortune.
Fortnite killed Team Fortress, Overwatch killed Halo and Minecraft was the new digital lego. None of them required such a skill to play or host the game. It was refreshing for it's time.
Not forgetting it was the upcoming of a new crowd of folk and as generations go, the old cards topple.
Who hosted an BBS when the internet came to be?
IRC today is pretty much stuck in dinosaur country and XMPP/Jabber never got a product which then was further nailed shut by Google & Co.
It was epic fun for the era but it had a death bed waiting.
When I was a kid playing Quake 3 I had different servers bookmarked, knew the players that ran the servers and the general crowd that hung around. Had friendships and rivalries.
Modern multiplayer you may as well be playing bots. There's no community.
They're just all single player focused.