Let's go back into history. Unless you understand the past, you'll never get the present.
Let's go back to the 40's, 50's and early '60's - especially down South. There were "Colored Only" and "White Only" water fountains and rest rooms. Blacks weren't allowed in certain hotels and/or nightclubs, weren't served at lunch counters, had to sit in the back of the bus. Many Black entertainers had to walk through the back of a nightclub. Black schools and schoolrooms were substandard to White ones. Blacks had to be escorted by the National Guard in order to attend schools, because they met resistance by that state's governor standing in the doorway. Blacks and Whites, were fire-hosed with the power so high it ripped their clothes off, and set upon by dogs, died fighting for the right to vote, shop, attend schools/colleges of their choice. If it weren't for them, many of us wouldn't have the jobs we have today.
Bottom Line: It's because as a people, we were sick and tired of being treated less than human, and denied our basic civil rights. Not to sound bitter - but put yourself in our shoes: would you like it if you were denied i.e., the right to vote/shop/live anywhere you please?
As for Ebonics - it shouldn't be taught. It will not help you in filling out a job and/or test application, converse during an interview or in basic conversation. Correct English wins out every time-and THAT'S what should be taught.