I think Kwame Ture put it best.

Dr. King's policy was that nonviolence would achieve the gains for black people in the United States. His major assumption was that if you are nonviolent, if you suffer, your opponent will see your suffering and will be moved to change his heart. That's very good. He only made one fallacious assumption: In order for nonviolence to work, your opponent must have a conscience. The United States has none.