Because there is no officially established extenuating circumstances recognized by the State Department. Until that happens, there is no grounds for a special visa. They just get in the back of a long, long line...
The visa procedure for South Africans traveling to the United States does not appear to be all that burdensome.
All the Africans have to do is go to the United States Embassy or Consulate, apply for a visa and hop on a plane.
https://www.intergate-immigration.com/american-united-states-visa.php