Problem is, most people are going to take one look at join-lemmy and not know what to do. It's so much easier to point people at lemmy.world or lemmy.ml.

And until very recently, small servers have had a ton of trouble finding communities on the big servers. That seems to be mostly resolved, though I still get community not found errors when I first navigate to a new community, but refreshing takes care of this. New users aren't going to know what to do with this.

Granted, many (most?) people seem to think these problems are a good thing because it keeps the normies out, but forums are nothing without people.