Language barriers. German feddit.org and Italian feddit.it are also outside. Partly that also might be a country thing. aussie.zone is also a bit outside. The other big group are defederated communities like lemmynsfw, lemmybear and hexbear.
Slrpnk is most English and many topics are relativly intressting to none instance users. So you get that. Besides a bunch of more solarpunk specific communities that is.
So federation seems to work rather well.