It was a while ago. Apparently they thought their vision was more to be a self contained forum than connected to everyone else and also that it was “safer”.
As far as I remember they couldn't manage all the problematic content, especially comments with the limited resources and bad moderation tools in Lemmy to deal with the huge amount of people from the biggest instance.
That makes sense. I recall some people saying it was contrary to the ethos of the Fediverse but I don’t blame Beehaw. It’s perfectly legitimate to use Lemmy as a self contained forum or to restrict federation as the admins see fit.