It seems ActivityStreams came first with name, and ActivityPub could only add a new property on top of that. (Especially since name is one of the few special properties that can also be a natural language map, and it seems preferredUsername is not)
Also, in our brief talks on Fedi, I know you do not have a goal to become ActivityPub compliant at this time. I was wondering if it is something being built towards on the long-term roadmap, even if not a priority anytime soon?