Board Thread:Theories and Speculation/@comment-32617252-20180317055942/@comment-26773508-20180317073602

I had always wondered about that. In Madara's case, the best explanation is that he wasn't aware of the conditions for the EMS until Izuna voluntarily bequeathed his own eyes to Madara on his deathbed to ensure that Madara would continue protecting the Uchiha. I think only then the EMS was unlocked in a long, long time, or perhaps even the first time in Uchiha history.

As for Itachi, that guy had no desire for such power. He was the ever-protective Nii-san and a servant of Konoha till his dying breath and beyond (as an Edo Zombie). I'm pretty sure Itachi was well aware of the possibility of this cheat code, but he neither desired it, nor was it of any use to him, since he was dying of a fatal disease anyway. His only goal was to protect Sasuke and Konoha.