Economist David Henderson explains:
Proponents of the minimum wage, when it was legislated in 1938, were disproportionately from Northeastern high-wage states where a minimum wage would be binding only on a very small segment of the labor force. They used it to narrow the differential in wages between the Northeastern states and the Southeastern states, where black men were a much higher fraction of the labor force and where the minimum wage would be binding on a much higher fraction. I posted about the role of Senator John F. Kennedy in the 1950s and his explicit statement that he wanted to hobble competition from black labor.
Much more at the full post here.