Historically, most blacks are Protestants, whose families were originally from the south. They really didn’t start to migrate up north much until the 1920s or so. And the south had VERY few Catholics down there to being with, much less those who owned slaves. Even when I moved to Virginia in the mid-60s, Catholics were only 4% of the population. One of the few places down south that DID have Catholics was Louisiana, and New Orleans in particular. Blacks are mostly Protestant, for that reason. The essentially got taught “Massah’s” religion, which was almost always Protestant of some sort. Mother KAtherine Drexel, who was the daughter of wealth from Philadelphia, had a specific mission for teaching to the Native American Indians and “Colored” children. See Xavier University, in Louisiana. Actually, if you look at the numbers, those 16 black bishops are ABOVE what you’d expect given the number of priests. So if you do happen to be a black priests, your odds of making bishop are much bigger - essentially a 5% chance, and there’s no way that holds up among white priests making that office. Also, to study and attain the priesthood requires a fairly rigorous mind. 4 years of college, plus 4 years of graduate theology. If you didn’t do a religious college aimed at the priesthood, add another year or so for formation. And those are diocesan priests. Other orders may have more stringent requirements. It used to be Jesuits took almost 14 years of study from age 18. So, frankly, work the bell curve in there too, because relatively fewer black men go to college than white men.
And do not forget that blacks in the South were Baptists and as anti-Catholic as white Southern Baptists. Add to that the racial bigotry of white Catholics. In New Orleans, I went to mass in 1954 at the Cathedral , and there was a mixed congregation, When I went to communion I walked directly to the Altar rail and knelt down. Then I realized I had knelt among several blacks, which explained the startled look on the priest’s place when he came to me.