In Japan, there is no culture similar to Reddit in the first place, so Lemmy has not been widely accepted, and there are very few users. I tried promoting it before, but it had no effect. Similarly, Mastodon groups are rarely used except for a few cases. Even if you come across Japanese users elsewhere, most of them can speak English and have used Reddit in the past.
Moreover, as is the case for me, most Japanese people do not understand any language other than Japanese—not even English. Because of this, even if they connect with other global servers, they cannot communicate due to language and cultural differences, which I believe has led to this situation.
Misskey is, of course, very popular since it was developed by a Japanese person.
I myself run a Misskey server.
Sorry, but I couldn't find any information about plans for Misskey channels to federate with groups.
Misskey's developer is not very proactive about federation, so if this is true, that would be great to hear!