The Cunninghams are poor but they are proud and won't take handouts. They are honest and just trying to make do. They are not part of the "upper social structure" of Macomb. I feel Lee chose Cunningham to be the "not guilty" verdict because he can more easily relate to the injustice of Tom Robinson's situation. He knows what it is like to be looked down on and he knows what the people of Macomb are like. Cunningham is probably the most upright citizen in Macomb next to Atticus. He was one of the leaders for the lynch mob but Scout shamed him into stepping back and leaving. He had time to reflect on what Scout said, and he listened to all of the evidence and made the right choice. "Walter Cunningham. Sr., is a member of a poor family who "never took anything they couldn't pay back." A former client of Atticus's, he paid for legal service with goods such as firewood and hickory nuts. After Scout recognizes him in the potential lynch mob and speaks to him of his son. he leads the crowd away from violence. "
Cunningham recognizes in Tom Robinson his own disenfranchised status in the town. He can relate to the prejudice against Tom's race, having experienced the same prejudice against his own class. Scout's recognition of Cunningham in the lynch mob would have reminded him of his connection to the Finches through the kindness to his son, and to himself, in former days. The novel's shifting contrasts between characters, emphasizing on one hand their differences (as when Cunningham is part of the lynch mob) and their similarities (as when Cunningham is reminded of his lowly status) is a theme borne out by Lee's choice of Cunningham as the one holdout in favor of Tom Robinson's acquittal. When Cunningham walks in Robinson's shoes, he can fairly judge him.