I'd say there's three main reasons:
1) Clever psychological manipulation. Iago knows exactly what to suggest, and when to suggest it. He says things briefly, succintly - those four devastating words "look to your wife" start a whole battery of thoughts in Othello's mind. He also (note the way he makes Othello force him to keep speaking, by making a suggestion and then dropping it) makes Othello demand to know things from him - allowing him to seem more honest and innocent. And because Othello thinks him honest, Iago can easily lie to him.
2) Othello's own insecurities. Othello makes a telling speech after Iago has exited, about why Desdemona might have cheated on him:
Haply, for I am black
And have not those soft parts of conversation
That chamberers have, or for I am declined
Into the vale of years—yet that's not much—
She's gone. I am abused, and my relief
Must be to loathe her.
Othello is a foreigner (note that Iago emphasises that he knows all about Venetian woman - implying that Othello doesn't), he is racially different, and he is older. He feels insecure, I'd argue, about each of this things, and Iago's suggestions activate that insecurity, which in turn defeats his better judgement.
3) Good luck. Iago does seem to have ideas and plans, but some crucial turns - Cassio getting as drunk as he does, Desdemona dropping the hankerchief - are just down to good fortune.
Hope it helps!
Iago is one of the most nefarious villains in all of Shakespeare because he seems to delight in his own selfish revenge and loves to do evil for evil's sake. Iago completely manipulates Othello because he manipulates situations, other characters, and even what Othello sees and hears.
Othello is a play of psychological manipulation. Iago first manipulates Cassio's watch so that Cassio's reputaion is erroded and his is "plumed up" in the process. Having disposed of Cassio's reputation in this way, he is free to change the Moor's passionate disposition by whispering such things as "I like not that" when Cassio and Desdemona are together. By "pouring poison" into the Moor's ear and letting O's imagination do its worst, is Othello completely ensnared.
Iago manipulates the Moor by orchestrating a discussion Iago has with Cassio where C talks of Bianca (but O thinks it's Desdemona), provides "ocular proof" of Desdemona's supposed infidelity, and pretends to be Othello's "blood" brother. In orchestrating the chance encounters, other characters' actions and speech, and through a manipulation of the passionate General himself is Iago able to "completely" influence Othello,