What reason does atticus give for defending tom robinson?

Because he feels that every lawyer has a case at one time in his life tht changes him, and this is his. He choses to defend him because he knows that no one would defend a black man, that he owes it to his race on apology for the unfairness that has been given to the black man from the dawning.

Atticus defends Tom because he can not defend himself and he knows he will not get a fair trial.