Problem is that a) new users don’t know that they can join communities across servers, and b) it is intuitive use start with the servers that a lot of people like.
Instance browsing and onboarding is probably the biggest challenge to Lemmy’s growth. The current experience either scares new people away, or encourages them to congregate on a limited set of instances.
If the registration process just picked a random instance for you, maybe something nearby, and assured new users that they can visit communities and interact with users across instances, very few would pick the biggest instance.