When Tom is being cross-examined by Mr. Gilmer he says, "I felt right sorry for her". Why was this a mistake? Why is it that "nobody liked Tom Robinson's answer"?

Tom was black, for him to ever feel sorry for a white girl was inexcusable and unheard of by the white population:

“Yes, suh. I felt right sorry for her, she seemed to try more’n the rest of ‘em—”
“You felt sorry for her, you felt sorry for he?” Mr. Gilmer seemed ready to rise to the ceiling.
The witness realized his mistake and shifted uncomfortably in the chair. But the damage was done. Below us, nobody liked Tom Robinson’s answer. Mr. Gilmer paused a long time to let it sink in.