It's because after the exploitation trend, the studios figured out that many of those same black people would also go to blockbusters like Jaws and Godfather so the studios were still getting those black dollars. Blockbusters became more and more important so then the excuse became, "Well, minority led films don't do so great overseas." So then their thinking was it ok to have black people in the movie but they couldn't be the leads because it would supposedly alienate part of the larger audience or even when the black man is the lead, he'll often be the only minority because adding a black love interest or support cast makes it a "black" film which is niche product to them.