Iago's scheme would not have worked without the underlying atmosphere of racial prejudice in Venetian society, a prejudice of which both Desdemona and Othello are very aware. Shakespeare's Desdemona copes with prejudice by denying it access to her own life: Her relationship with Othello is one of love, and she is deliberately loyal only to that.
Othello, however, is not aware how deeply prejudice has penetrated into his own personality. This absorbed prejudice undermines him with thoughts akin to "I am not attractive," "I am not worthy of Desdemona," "It cannot be true that she really loves me," and "If she loves me, then there must be something wrong with her." These thoughts, inflamed by Iago's hints and lies, prevent Othello from discussing his concerns and fears directly with Desdemona, and so he acts on panicked assumption. In order to survive the combined onslaught of internalized prejudice and the directed venom of Iago, Othello would have had to be near perfect in strength and self-knowledge, and that is not a fair demand for anyone.
