lemmy.ml was a sort of prototype made by the devs of the lemmy software. It wasn’t really meant for widespread public adoption. So it makes sense that they went with a free domain.
I get a little over 10 dollars a day. Also, while my country has an excellent, free system for internal financial transactions, any international transaction will be (a) complicated and (b) expensive.
I’m not saying I wouldn’t pay USD1USD0 for a website, but I sure wouldn’t do it for a hobby one.