Why is it significant, in Alexander’s view, that the U.S. Supreme Court prevents defendants from claiming racial bias in criminal justice cases?

Why is it significant, in Alexander’s view, that the U.S. Supreme Court prevents defendants from claiming racial bias in criminal justice cases? (See examples on pgs. 109, 122, and 131)
To what extent does Alexander use the criminal justice cases to argue convincingly the effects of racial bias in U.S. law?