Why doesn't Iago tell Othello about Desdemona's and Casio's affair?

Answer 1

Because Othello would never believe him if Iago just told him. Iago wants Othello to believe that Desdemona and Cassio are having an affair (which they are not), and so goes about it in a much more devious and effective way. He starts off making little comments under his breath, like "Oh, I like not that." and then the more Othello presses him to tell what he is thinking, the more reluctant Iago appears to be. He says he doesn't really know anything, it's probably really innocent, he doesn't want to get anyone in trouble, BUT . . . and then relates some mildly suspicious incident. As Othello gets hooked and forces Iago to tell more and more, Iago makes up more and more increasingly suspicious incidents. Othello believes him because he seems reluctant to say anything, and also because everyone calls him "Honest Iago". This is textbook con artist technique. And Iago uses it over and over, gulling people right left and centre throughout the play. Shakespeare's depiction of this kind of deception is masterful.