Beehaw became a hub for the exodus largely through a misunderstanding—they were already active when Lemmy exploded, so a lot of the exiles found their way into their communities, but Beehaw's culture is in a lot of respects incompatible with Reddit's. It was only a matter of time before they decided preserving their community was more important than sheltering all of Reddit's exiles.

I highly recommend reading Beehaw's history that they link from their sidebar. It helped me better understand the value proposition of the fediverse, and goes a long way towards explaining their perspective on the necessity of defederating the large instances:

https://beehaw.org/post/107014