This is an interesting question. Is Othello or Iago the main character? I think the story is essentially about how a man falls in love with a beautiful woman, marries her, then becomes insanely jealous and murders her. Iago causes all the trouble, but Othello does all the suffering. Othello is the starring role; Iago is a supporting actor. I have always thought that Shakespeare would never have written the play if he hadn't been enamoured with the visual and poetic possibilities of the last act in which a man murders his wife and loves her deeply even as he is murdering her. That soliloquy which begins
Put out the light, and then put out the light.
If I quench thee, thou flaming minister,
I can again they former light restore,
Should I repent me ...
Is one of the most beautiful things Shakespeare ever wrote. Iago's inferior importance is seen in the final act when he is being taken away to prison and looks like a common criminal getting his just reward.
I don't think there is any rule that says that plays should be named after the most important character. Even Shakespeare had The Tempest, A Midsummer Night's Dream, As You Like It, and others.