First of all, Orestes is the protagonist because he gets the action going. It is his return from exile that upsets the status quo that has reigned in Argos since the death of Agamemnon. Don't forget that it was Clytemnestra's prophetic dream about her son's return that made her send out Electra and the Chorus women to make libations at Agamemnon's grave in the first place. (This points to the fact that the gods could really be behind everything, but we won't get into that right now.) After getting the ball of the plot rolling, Orestes also has to keep it in play. He is the one who must infiltrate the palace, find out his father's killers, and kill them. They don't call the trilogy the Oresteia for nothing.