Seems like they lacked the fuel? I don't have a globe handy, but a quick glance at Google Maps suggests Boston is slightly closer to Johannesburg than Atlanta is.
In December 2019, a deadly breakout of novel coronavirus SARS‑CoV‑2 spread and was declared a global pandemic. Recently, the WHO declared one particular variant first discovered in South Africa as a "variant of concern", and the US imposed a loose "travel ban" on noncitizens from this country. This is one of the last flights out of South Africa before the ban is set to be enforced.