Blacks from the U.S. can generally trace their roots back to Sub-Saharan west Africa.
Not too many distance wold-class athletes can trace their roots to that part of the world.
Those who can, tend to dominate sprinting events and sports that require "explosive" bursts of
speed: 100m dash, soccer, football, etc. I doubt we'll ever see a U.S. black win or even top-ten at the TDF.