Onboarding. The fact that you have to choose an instance to join while creating an account is essentially forcing people to make a decision for which, unless they’ve done some reading, they’ll have no idea of the implications. It’s such a weird concept for new users - they have to know about a thing before they’ve had experience with a thing.
Even if it doesn’t really matter which instance you begin with, the experience will be different, and there’s a sense of “pressure” at the point of signup, which doesn’t exist outside of the Fediverse.