While Bonfire is a common place for some players to go, I think it seems more popular just because it's a smaller map. It's probably got the same amount of people as Fluorite Plains, but on a small island. Thus, it makes it look like there's more people.
But added onto that, if a player accidentally stumbles upon Bonfire, and can find more players that they relate to in that map more than anywhere else, I'm sure they'd be eager to stick around there. I know I would be!
1) Its small 2) Normally the MC and AC are there 3) Lion prides are popular there too. I normally see a lot of lions there I guess cause of the rock the sticking out of the cliff kind of reminds people of pride rock