Ego leading to misaligned goals. Their entire goal ending the franchise seemed to revolve around generating speculation and mystery. Like, this was the basis for the ending. Hudson was desperate to have the franchise end in a way that would make it very memorable (which I can understand), and Walters wanted to harbour mystery like a lot of hard sci fi endings do.
Rather than sit back and try to write a concise, believable and complete narrative, they let their goals take first preference at the expense of everything else. They didn't write a story, they wrote a disconnected end sequence with its own specific intentions.
