Thneitis

Just finished reading and want to write my guess somewhere where I can reference it before the new book drops. I believe that they thought if they stayed as radiants it would usher back the real odium. They all abandoned their oaths after the false desolation because they thought them being there would lead the singers to keep trying to bring odium back. That explains why the sky breakers stayed. They were the order that was tasked with ensuring no other radiants ever came back secretly, so that odium wouldn’t return.