Black people don't own as many businesses because white supremacy leaves them to start out a low-income level. This makes it hard to accumulate the economic and intellectual capital to start a business.
How to fix it? That's the hard question. There's governmental business loans for minorities out there but it's not nearly enough. Personally I'm in favor of reparations to start.
Fact of the matter is that there were many roadblocks put on blacks in America post reconstruction. The main goal of all of those was to deny and hinder the process for black americans to achieve wealth on a collective scale period. There were more black businesses before segregation than there were after segregation.
Through integration and crises affecting low income earning areas predominantly occupied by blacks, the quality of the school systems in general became poor as they became underfunded thus making it hard to accumulate the education necessary to learn skills to become employable or a trade. Then there was the export of industry in America which kept many middle class black families in tact as this was usually a go to for most black men and women in earlier decades.
Not excluding the drug wars and the crack epidemic which created epic amounts of crime in those areas which led to many black men and women going to prison leaving families devastated amidst the violence it left in it's path.
But it's pretty much all orchestrated by white supremacy on a political, social, and economic scale because black people have never had the power in the history of American life to control the strings being pulled by the powers that be that seek to maintain blacks as a permanent underclass in a capitalist system designed to do just that.
There's more to it though...but that's the main connection between all of this. Add zoning restrictions, denial of loans by banks or the assignment of such with high interests rates that leave them impossible to be paid, biases amongst consumers, producers, costs/production ratios, lack of access to production facilities for textiles/electronics, not to mention territory in neighborhoods having outsiders and corporate retail shut out the competition, and you have a very tough situation for the average black person to start a business on their own let alone succeed. Because most businesses fail in their first year.
It's apart of a fractured psyche placated on all americans to doubt themselves into going into business for themselves and to stay complacent working for someone else to survive.