In addition to Iago's "proof" of Desdemona's infidelity, there is the pervasive notion that no one in the play perceives Othello entirely as an equal. At the beginning of the play, when Iago and Roderigo are informing Desdemona's father, Brabantio, about the elopement of Desdemona and Othello, they euphemisms they use are completely dehumanizing. It is obvious that Iago and Roderigo believe Othello to be somewhat "sub-human." This is mirrored by Brabatio's horror at the marriage. Brabantio has had Othello at his house as a guest many times. Othello is acceptable to Brabantio as a houseguest but certainly not as a son-in-law. This lack of complete acceptance has shaped Othello's character. It is obvious that he expects to be rejected. He is too ready to believe the worst of Desdemona without her word.
Othello believes Iago over Desdemona because of Iago's "proof," such as Cassio leaving Desdamona very quickly and then right after Desdemona's pleading on his behalf. The hankerchief that Iago plants with Cassio, that once belonged to Desdemona, only cements Othello's trust in Iago. ALl of this "proof" of course is of Iago's creation, none of it is actually true. It is for these reasons that Othello believes Iago rather than Desdemona