That is because language is dynamic and grows to the convenience of the people speaking it. The amount of the divergence will be proportionate with the period of their isolation. If you question for more than a millisecond that blacks and whites in the United States have been isolated for generations, look at the U.S. Census data, or even the Jim Crow laws of the south. Ebonics was therefore born of the southern dialect and fostered by enslavement and segregation over a period of more that 100 years. It became widely popular with the black films of the 70's.
Shaft is a bad mutha, shut yo mouth.
One might argue that the isolation ended with the advent of integration in the 60's. If it did, that would still leave more than 100 years of divergence. But that isolation has continued by conscious housing and school patterns, controlled in part by bank lending policies prevalent until the 90's. It could be argued that television should have blured the distinction by offering a widely available standard reference for the language. Not so, look at their programmatic content.