Board Thread:Theories and Rumors/@comment-34113480-20180825003103/@comment-35245651-20180825070153

If we want to have a nice, sensical explanation, then we could say that the Cloud Kingdom was a secondary realm created by the First Spinjitzu Master (explaining why it is inhabited by humans and not Djinn, anthropomorphic animals, or any other race), whose sole purpose is to write the destinies of only humans (thus allowing the prophecies of the Green Ninja and Ultimate Spinjitzu Master to come true no matter what). That would also explain why Fenwick didn't just change Morro's destiny, as Morro and the rest of the ghosts as well as the Preeminent resided in the Cursed Realm, rather than Ninjago. He couldn't change the destiny of a ghost.

However, that's obviously not canon, but I'm sure someone could ask Tommy or whatever.