According to "Letter from Birmingham Jail", why is King “so greatly disappointed with the white church and its leadership”?

Overall, he argues that there are “creative extremists,” those who wish to create and improve rather than destroy (180). Dr. King confesses his disappointment that white moderates have not made this distinction, but considers that whites cannot truly understand “the deep groans and passionate yearnings” of blacks. He does admit that some whites in the South understand the cause and are committed to it, and lists some of them. He praises these people, many of whom he speaks of in generalizations, for marching and suffering alongside blacks who are taking action. He rebukes white church leadership for not taking on an active role in the cause of black Americans.