Why was othello eventually defeated and deceived so totally by iago?

The short answer is that he was too-trusting and had far too much faith in iago's apparent honesty, but iago's military status and othello's outsider status also factor in. iago being high-up made him look like someone trustworthy and othello, being an outsider, quickly believed iago and trusted in his understanding of how things worked in an unfamiliar land for him.