The uchiha clan was planning a coup d'état, right? So they wanted that the uchihas rules the village... what if hiruzen made Itachi hokage... wouldn't that be good? Itachi had a great will of fire, he would've done the best for Konoha AND it means that an Uchiha had the might... then the uchihas wouldn't plan a coup d'état, because one of them was hokage, and it wouldn't be in the disadvantage of konoha, like I already said, Itachi was very strong, smart and had a great will of fire... so why didn't hiruzen make itachi hokage?