In general, people train for the jobs that are likely to be open to them. They see what other people like them are doing for a living and they expect that they, too, will occupy such jobs. Therefore, they train for those jobs rather than for other jobs that they do not think will be available to them. This is why African Americans in general trained to be laborers and workers rather than bosses.
For most of our history, African Americans were treated very badly. There were hundreds of years in which many African Americans were enslaved. African American children who were growing up as slaves would not have been trained to be bosses. Instead, their owners would have trained them to be laborers and workers. After the era of slavery, most African Americans lived in the South and were heavily discriminated against. They were generally pushed into menial jobs rather than white collar jobs or jobs as “bosses.” Therefore, most blacks trained for such jobs. We can see this, for example, in the attitudes of people like Booker T. Washington. They felt that African Americans should accept the place in society that whites had imposed on them. Therefore, they should get vocational training rather than a sort of education that would train them for “better” jobs.
African Americans have typically trained for jobs as laborers or workers because those were the jobs that were available to them. This was a rational response on their part to living in a society that did not treat them as equal to white Americans.