Yeah, I guess people have a reflex to always go wherever is the biggest (which doesn’t really make sense in the Fediverse).
Mastodon is different, though. Mastodon.social is the default instance and is heavily suggested by the company, while join-lemmy.org lists instances randomly by default. There must be something that inclined users to join it, considering that it gained enough momentum to make up more than half of Lemmy users (not counting alien.top).