It has to do with how federation works. There's no federated broadcast to all instances in the threadiverse when a community is created. Each instance has to be taught about each community via a search. I think this was done to prevent broadcast storms due to community creation, though in normal circumstances community creation is not a high frequency activity. I think the world would be better off if community creation broadcast but was rate limited.
At any rate, in the meantime search for communities at lemmyverse.net. It's way WAY better than the native community browser. Then just use the native community browser to find an sub the specific thing you found in lemmyverse.