The main reason for a later priority on local split screen from my understanding is that it requires other systems to be completed first.
For local co-op they need the controller GUI to be completed. For the controller GUI they need the standard GUI to be completed. Finally for the standard GUI to be completed they need all the game systems to be implemented, which is not the case yet. A short list off the top of my head; crafting, game Master, skills, ect.
Others can correct me if I missed anything.
