That is not surprising for a site of 50,000 people(though some niche Lemmy communities may also be in the wrong instance
I feel like this isn’t as relevant as people think. A community on a small instance may take some additional legwork initially but once one person from a large instance joins a lot of people who sort by all will see it in that other instance.
I imagine people who sort aren’t sorting by subscribed are likely sorting by all rather than local because of how spread out popular Lemmy communities are.
I feel like a better solution is to link to other communities when posting in an already niche community. For example if I want to share a video about the lore of the Fallout games mentioning !fallout and !fallout at the bottom so more people are aware of them and they can both benefit.