The answer is: the absence of labor unions in the South
Following the world war ii, the majority of wealth in this country is located on the northern states since they are more industrialized compared to the southern states.
The more advance industrialization on the northern states make northern states' factories generated a lot of money from producing war-related products. Because of this the majority of job opportunities in United states also located in northern states.
Not only the southern states have less jobs, they also treated minorities unfairly. This led many African Americans decided to move to northern states. The existence of labor unions in the south actually help the workers get better working condition. It influence southern people to stay, not to leave.