The Kings watch from afar, like bosses. They wouldn't send their ranks out when they can take advantage of the expendable Devils. They tested SIVA's augmentative abilities in the Devils, like lab rats. They needed to see what it would do to Fallen DNA, without risk of damaging a King. After Aksis, who was most likely ordered by the Kings to become so powerful, they've seen its capabilities and can now augment it even further. The Kings aren't present in the plaguelands as encounters, because being in such a place is beneath them. But surely they've made their presence known by their banners and connection to the Devils. What the Kings have planned for us will make Aksis look like a true puppet. Destiny 2 will most likely focus greatly on the Kings, Savathūn, and the Cabal Empire.