This is something that lemmy devs need to better address. This is an “Eternal September” kind of situation. People (me included) are not used to the fediverse. They think you can participate only if you’re in that instance. And people want content, so they think "why’s the instance with most people? Ahh lemmy.ml? Cool, let’s join.
Go to the bottom of the page, click on the Instances link and you’ll se 2 lists, all federated instances and all defederated instances.