Iago always uses the characteristics of his foes against them. In the case of Othello, clearly his biggest goal of manipulation, he finds a way to use his confidence and his "warlike" abilities against him. Having ended up on Cyprus and without a foe, Othello loses his great confidence in his abilities as a soldier and a general.
By using the fact that Othello is not as confident in his ability to actually win the heart of Desdemona, Iago prey's on Othello's insecurities. He knows that Cassio is the darling of the royalty in Venice and so he would be the likely opponent in the contest for Desdemona, also royalty whereas Othello is an outsider and a moor.