You have to remember that until recently, there was sub 100 daily users, this wasn’t a big platform, and it wasn’t just lemmy.ml, but a bunch of <10 user instances.
It wasn’t worth paying for a small side project until it wasn’t and at that point it was too late, plus who would have predicted that the gov of Mali would forcefully take back all of their domains?
i mean, good point on the project size, but buying a domain is honestly such a basic thing that it still feels like a weird result on that equation to me. for fmhy.ml, specifically, i understand their choice, since pirate sites do tend to be quite nomadic with their domains, and the fediverse being so domain-specific is a new thing.
still, domains are hella cheap. the $10/yr figure i quoted is for a “serious” one on one of the major tlds, you can get away with much less if you’re willing to go for a somewhat more niche but still reputable one. especially one that’s longer than two characters.