Genetics and body proportions.
Blacks, on a genetic average, tend to have longer legs and a short torso, compared to the white genetic average of a longer torso when taken as a legs to torso ratio.
This imparts a few, of a sort, physics-based variables that swing explosive, short-distance sprints toward those with the highest leg:torso ratio. Center of gravity and proportion of where mass is located, as well as stride and explosive muscle contractions.
But it's a hindrance to endurance events. Energy must be conserved so stride length is different, and taking shorter strides when you are better geared towards longer more powerful strides, is not energy efficient. The change in center of gravity may also impact how momentum and energy come into play for endurance events.
And unless you are a freak, even with the best professional training, you really cannot compete against a person with a higher legs:torso ratio if you have a longer torso and comparably shorter legs.
For whites, it is a bit rarer for us to have that ideal sprinters proportions. And even fewer will actually get into competitive sprinting to test and hone their abilities.
I reckon, if you measure most blacks who compete in endurance events, they likely do not have that "ideal" sprinters body.