I think Iago’s character developed into a protagonist as time went by. People were more obsessed with “famous” characters. Call them the celebrities of the day. He still had to sell tickets. But time evolves our reading of his work, characters.
While Iago is actually the main character, it was the Othello’s otherness that made it easy for Iago to manipulate him. It was the fact that Othello wasn’t 100% sure about his relationship of Desdemona that let Iago’s words worm into his head.