Possibly related bug report: https://github.com/tgxn/lemmy-explorer/issues/191

I know that other services, like the trending community bot, that recent had issues in the same vein. The trend bot only showed like 3 communities, and it normally shows about a dozen.

So, there is likely a federation issue, or something wrong with however these services get their data.

MrKaplan mentioned that there were a ton of "spam communities", so possibly they have since been removed. They weren't real anyway. https://lemmy.world/comment/13613002.