Federation question: Why can I subscribe to some lemmy.ml communities but not others?

I noticed that at the list all communities page I see many communities from lemmy.ml, but not all of them.

The two I happened to notice were "covid" and "crypto". I'm new to Lemmy and don't know what would cause this. I found this earlier question but the answer implies at least there's a way for a lemmy.ca user to subscribe to the community in question, and I can't figure out how to do even that.

I tried going directly to https://lemmy.ca/c/coronavirus@lemmy.ml but I'm taken to a page that says "Couldn't find community.".