In The Great Gatsby, why do you think Tom picked Myrtle to have an affair with?

It is likely that Tom picks Myrtle because he thinks she will be an easy woman to control. She is not wealthy, powerful, or socially connected, so she doesn't pose much of a threat to him. Myrtle also readily accepts Tom's fabricated story about being unable to divorce Daisy because "she's a Catholic."  

Myrtle is also a relatively inexpensive mistress. Tom buys her a puppy for $10 and sets them up in an apartment in a neighborhood in which the rent wouldn't be exorbitant. She is apparently not savvy enough to understand Tom has had other mistresses from the working class (such as the chambermaid at the hotel where he and Daisy stayed three months after their honeymoon) and that she is unlikely to be his last mistress.

Lastly, Myrtle is a woman who seems comfortable in her sexuality, and Nick describes her as stout and sensuous. Myrtle shows no embarrassment about disappearing into the bedroom with Tom while Nick sits alone in the living room of the apartment.