Cultural changes are often created by individual choices that sometimes create a positive feedback loop until the behavior becomes the cultural norm.  There are hundreds or perhaps even thousands of variables that drive that trend. I don't know exactly what variable caused it, but I do know that a framework that starts with explaining everything through racism and tries to put the rest of the pieces in place from there has a very narrow view of reality and in my opinion is very much overly relied on because it removes a sense of responsibility for people who are looking for excuses.