Relative obscurity is an aspect of it for sure, and I imagine networking effects are a big part of it as well. Accounts with a lot of followers tend to drive where people migrate. When a big account leaves twitter to bluesky they can pull thousands or even millions of users along with them. Meanwhile, people moving to Fediverse end up having to abandon a lot of the people they were following as a result. It's also a lot easier for an account to grow on a centralized platform which is another reinforcing factor here.