Part of it was the fact that Hollywood treated it was just another trend. And seriously...the way some of those movies were made, it was. In the 70's at the same time you had cheerleader sex comedies, horror movies, and even what they call "Hickploitation" movies. It was B-Movies/exploitation films that helped saved the industry in the early 70's ( including Porno Chic). You get to the late 70's-early 80's Mainstream Hollywood figured that black audiences will go to mainstream movies if there was at least one black person in the cast. This is how we get the "Black Best Friend" trope.