Requiring a local subscriber for federation to occur is tough on smaller instances. When I started up the !manga@ani.social community, I had made a bunch of alt accounts across different instances to subscribe to help with discoverability. There are some tools people have built to help seed communities between instances (https://lemmy-federate.com/ is one I have bookmarked), but it is definitely a shortcoming of the federated model. I wish that lemmy was more capable of fetching historical content from a different instance.