Another reason why it was so easy for Iago to manipulate Roderigo is because Roderigo is very gullible. In Act 4, Iago tells Roderigo,

"I grant indeed it hath not appeared; and your suspicion is not without wit and judgement. But, Roderigo, if thou hast that in thee indeed, which I have greater reason to believe now than ever-I mean purpose, courage, and valor this night show it. If thou the next night following enjoy not Desdemona, take me from this world with treachery, and devise engines for my life"(A.4,S.2,P.220)

At the moment, Roderigo is so convinced that he'll have Desdemona if he listens to Iago because Iago tells him that his judgement and suspicion shows how serious and willing he is in order to get what he wants, and he basically promises him that Desdemona will be his by tomorrow night. Roderigo, being gullible as he is, believes that he can actually win Desdemona tomorrow night after getting rid of Othello. Roderigo would have to be very gullible to believe that, because Desdemona is so in love with Othello that she ran away with him behind her fathers back, and Roderigo still believes that she'll react completely normally to Othello being gone and will run into his arms. Many times in the real world, someone is taken advantage of because of their lack of intelligence.

Roderigo helps start a fight with a drunken Cassio, stabs Cassio, loses all his money and sells his land thinking all this will bring him Desdemona but it doesn't and he STILL trusts Iago and keeps falling into his traps. You'd have to be very gullible and stupid to keep sacrificing so much for so little then realizing you're not getting anywhere, yet still repeating the same mistakes.