what event in US history might Dr. king be referring to when he says "the negro's basically mobility is from a smaller ghetto to a larger one"?

Luther KIng referred to the Proclamation of Emancipation that in 1963 when he pronounced the speech called I Have a Dream, this fact was 100 years old. But after a century they faced a harsh reality, that the black was not yet free. Some ask those who fight for civil rights that when they are satisfied? and one of his answers was: We will not be satisfied as long as the basic mobility of the black is from a smaller ghetto to a larger one.