I joined Lemmy before federation worked, which is why I joined the “flagship” instance. In addition, other instances are often more localised, theme-specific or very political (Lemmygrad, Wolfballs). And some instances are poorly guarded outposts via which the troll incursions are coming in. Mind you, the admins there are probably good people but it does do little good to the name of those instances (and they risk defederation). Maybe if Lemmy as a whole grows people might successfully spread, but for now most would stick to the bigger instances with the most topics and the best moderation.

Edit: I interpreted the question as specific to Lemmy, even though the topic is in /c/fediverse.