See, Lemmy caught on with certain groups very early on. The main Lemmy instance has always been left-leaning. But one group that really latched on to Lemmy from the extremely early days was tankies.
These guys have their own community, "Lemmygrad" - IIRC, their servers are even based in China, or they claim to be. They were the most active users of Lemmy before Reddit shot themselves in the foot, and their influence has been felt across a lot of the other instances. If you ever went to GenZedong on Reddit, you get an idea of what they're like. (There's even a version of that subreddit on Lemmygrad.)
One reason why I switched to Beehaw is because they've blocked that instance. If you're on the "main" lemmy.ml instance run by the developers and you go to your version of /r/all, you'll see posts from Lemmygrad show up pretty frequently - because Lemmygrad still has a fairly large community, and they're still pretty active relative to the size of Lemmy itself.
If people go to join-lemmy.org like they're "meant" to, they'll almost certainly see Lemmygrad at the top of the instance list. Anecdotally, a lot of Redditors see that and get turned off, thinking this place is full of tankies. (And, to be fair... before everyone from Reddit started showing up, a lot of Lemmy was full of tankies...)
Even on Lemmy itself, there's a lot of people who dislike that side of the project. They're authoritarian at best, and genocide deniers at worst. And the people who created Lemmy are at least sympathetic to them, even if they don't outright endorse it publicly.
But. Lemmy is open-source. No one person or ideology can control the project, as it can always be forked. Even the creators of Lemmy itself have no control over what happens at Beehaw. So steering people away from the tankie side of Lemmy and towards a more, uh, "normal" instance is for the best, especially since this place needs to grow if it's to be taken seriously as a Reddit competitor.
Beehaw has chosen to step away from that drama by explicitly banning that instance, hence why I've been sending people on Reddit here instead of mentioning join-lemmy.org.