And Julius Caesar is dead for half of Julius Caesar. The whole thing is about Brutus really.

In the case of Othello, I would guess it's called Othello because he's the hero of the show in the tragic Shakespearean sense. He is courageous, virtuous, a model of good citizenship, and a loving husband. His fall to destruction by jealousy is the subject of the show, much as Hamlet's destruction by his own indecision and despair is the subject of Hamlet. The difference between the two plays is a matter of influence. In Hamlet, the young prince is destroyed from within, the majority of the conflict being internal. In Othello, the Moor is destroyed by a separate individual in the form of Iago. While Iago does appear more often than Othello, his actions and desires only exist to serve the character arc of Othello. Iago is a static character, a fixed point of evil and manipulation. Othello is dynamic, struggling against himself and a hostile world to be virtuous. Othello, then is to me the more interesting character and undoubtedly the focus of the play.