Because they didn’t turn on federation until last year, and at that point it was still limited to fewer than ten users per alternate server, and you had to manually request federation through a Discord server from an actual human. This year they’ve automated the federation process, but you still have to start with a tiny server, and they claim they’re going to raise the user limit gradually as new servers remain federated with the main server.

But yeah, the upshot is bsky.social has 13 million users, and there are no other servers with notable numbers of users. That’s a pretty notable difference from ActivityPub.