I reckon it's the lack of an (active) admin. They had issues with spam and worse from other instances, so I assume they just don't want to connect to any instance which doesn't have an admin who would be able to immediately stop it. It's a real shame for smaller instances like us, but it does kind of make sense for them.
If there's no captain at the helm then eventually this ship will run aground. We don't need to immediately jump ship, but signing up with another crew might be necessary.