Because lemmy.ml is the official instance meaning it was created by the developers of the people who created the backend for lemmy as well so people assume it would be the correct one to join.

Thankfully fediverse doesn’t work like that and in a few days I expect users to be spread around in instances more evenly.

and they don’t show up in the lemmy community browser

Communities are not visible to other instances until some user manually adds them.

If you are on one instance and you have the link for a community of another instance, that does show in the first, just copy the entire url of the unknown community, paste it in the search field of the instance that doesn’t know about it and search for it. It will NOT show up the result, but you can then delete the url and search again by its name. You will see the community now appear because you just indexed it.

I just learned this today, so I am going to spread it wherever possible.