Some other contributing factors that haven't been mentioned here yet:

They were their own instance for over 3 years before joining federation. That's 3 years of growth that all the new big instances from rexit didn't have, so when they joined, they were INSTANTLY one of the loudest voices in the room.

Their original goal for federating was to actively spread their ideology, and they were VERY aggressive and trolly about it (as they are about basically everything, that's essentially their inside joke). This ended up generating a lot of extra moderation work for instances and communities that have rules against being actively disruptive. There was eventually enough communication between admins and mods across hexbear and other instances that they toned back most of the trolliness, at least outside of their own communities.