why do you people have trouble with Atticus defending Tom Robinson?

I'm not sure what you mean by the question. Are you commenting on the book or the reader?

People have trouble with Atticus defending Tom Robinson because Tom Robinson is a black man and Atticus is a white man. At this time in America, Alabama is still segregated. White sentiment dictates that a white person "... couldn’t possibly be expected to take Tom Robinson’s word against the Ewells'" becuase Robinson is a black man and Mr. Ewell, and Mayella Ewell, her accusers are both white (Chapter 8).