Why does the author focus on atticus and not atom robinson

I am assuming this is about the book To Kill A Mockingbird. First of all, Atticus is the main character in this book. Symbolically speaking, he is the killer of racial discrimination, and is helping Tom in court. Atticus stands up against what is wrong, and tries to do what is right. Tom Robinson is just another character to symbolize the racism that was happening at that time. Atticus is the more developed and important character.