I think that it was also influenced by it being the developer’s instance (thus kinda had some official-ness to it, combined with the domain name reflecting the name of the service) and people wanting to just give newcomers an easy onboard by directing them straight to a link rather than to join-lemmy where some people get confused about choosing an instance. I had seen it in a few subs on reddit where it was just people putting lemmy.ml rather than join lemmy to give someone alternatives to reddit.

If you imagine you’re targeting reddit users looking for an alternative and you already know that some people get confused by choosing instances, you’re potentially more influenced to just give them a link to one of the instances. Doesn’t really matter where they sign up to an extent since it’s all federated, just need to skip them past the part that might confuse them. Then if you’re doing that, you’re also making it more familiar to things they understand by using a domain that looks official. If you say, “Use Lemmy” and then put “beehaw.org”, some might question why it’s called Lemmy but the website is beehaw. You don’t go to reddit.com but call it Linkit, so it may have seemed better to just use lemmy.ml to tell people to use Lemmy. Of course if you onboard people through join-lemmy then it better contextualizes names of instances that don’t have lemmy in them, but I think people were just trying to find ways to spread the word without potentially confusing people with the federation aspect.