Why do you believe Iago chose Cassio as the pawn to carry out the deceit against Othello?
By definition, a pawn is someone who is manipulated by another for that person's personal gains. One could argue thatin Othello, Roderigo is Iago's pawn, since pawns are generally viewed as weak people (and in the case of Othello, Roderigo is desperate and weak) and since Roderigo knowingly carries out Iago's dirty work in the hopes that doing so will win him Desdemona's love. Roderigo does whatever Iago tells him to do, which shows his inherent weakness as a character.
Cassio, on the other hand, is also manipulated by Iago--although Cassio doesn't realize it. Iago recognizes that Cassio is a well-respected, good-looking, and trusting person, and Iago is able to use Cassio's strengths against him. In his soliloquy in Act 1, scene 3, Iago plots to frame Cassio and Desdemona:
He hath a person and a smooth dispose
To be suspected; framed to make women false.
The Moor is of a free and open nature,
That thinks men honest that but seem to be so;(410)
And will as tenderly be led by the nose
As asses are.
Here, Iago observes that because Cassios has a "smooth dispose," and because Othello is so trusting, it won't be difficult for Iago to fabricate a story that pits Othello against his own wife and his lieutenant.