Probably because Didio didn't want a reboot or resetting of the DCU, or even just any future direction of DC, to be grounded in ideas not in synch with his own personal vision, let alone in synch with Johns' vision. Recall that about as soon as Johns left whatever position he had with the comics to focus on the films that it appeared as though Didio was moving to undermine "Rebirth" as soon as it had started.