The answer comes buried at the end of the piece:
Although many black families have moved up to better-paying jobs, they begin with fewer assets, such as inheritance, on which to build wealth. They are also more likely to have gone into debt to pay for university loans.
“African-Americans, before the 1960s, first by law and then by custom, were not really allowed to own businesses. They had very little access to credit. There was a very low artificial ceiling on the wealth that could be accumulated. Hence there was very little, if anything, that could be passed along to help their children get to college, to help their children buy their first homes, or as an inheritance when they die,” said Shapiro.
Since the 1980s, US administrations have also geared the tax system to the advantage of the better off. Taxes on unearned income, such as shares and inheritance, fell sharply and are much lower than taxes on pay.