Atticus has many reasons for accepting the judge's request to defend Tom Robinson. Tom and his family are members of Calpurnia's church, and Cal has no doubt put in a good word for him. Tom's accusers, Bob and Mayella Ewell, represent the family who has been "the disgrace of Maycomb for three generations." Bob is one of the few people for whom Atticus shows contempt: He probably knows that Bob has beaten Mayella before, and Atticus probably suspects that Bob beat Mayella, not Tom. Additionally, there are no other witnesses to the attack and no medical evidence of rape. Atticus reluctantly accepts Judge Taylor's order to take the case: " 'You're It,' " he tells Atticus. Atticus knows that he can refuse but that Tom will not receive as strong a defense from a court-appointed attorney. But above all, Atticus has Jem and Scout in mind when he agrees to defend Tom.
"... do you think I could face my children otherwise?... I hope they trust me enough." (Chapter 9)