Why does Atticus think juries unjustly convict innocent black men?

From the text:

“If you had been on that jury, son, and eleven other boys like you, Tom would be a free man,” said Atticus. “So far nothing in your life has interfered with your reasoning process. Those are twelve reasonable men in everyday life, Tom’s jury, but you saw something come between them and reason. You saw the same thing that night in front of the jail. When that crew went away, they didn’t go as reasonable men, they went because we were there. There’s something in our world that makes men lose their heads—they couldn’t be fair if they tried. In our courts, when it’s a white man’s word against a black man’s, the white man always wins. They’re ugly, but those are the facts of life.”