There have been a ton of great black speakers. Frederick Douglass, Martin Luther King, Jessie Jackson from the USA, and many more from all over the world. Verbal speaking is a highly valued component in black culture, so why is it that young African-American children are described as "verbally deprived"?

The linguistic inferiority principle states that the speech of a socially subordinate group will always be interpreted as inadequate by comparison with the socially dominant group. In other words, when African-American speech is compared to the middle class American average accent, it will be considered deficient.

A ton of false explanations are out there, including the fact that different races automatically have a different dialect, arguments based on anatomy like lip size, and many more. 

All evidence indicates that black culture is highly verbal, but unfortunately relationships of social and political inequality can lead to the dismissal of reality. Rather than being labelled as verbally deprived, they should be thanked for adding many phrases, words and ways of speaking to the English language that we use every day.