First COVID-19 vaccine trial in South Africa

Yesterday, 24th of June 2020, it was reported that the first COVID-19 vaccine trial was carried out in South Africa.

Volunteers received injections developed at the University of Oxford in Britain. According to Yahoo news;

South Africa has nearly one-third of Africa’s confirmed cases with more than 106,000, including more than 2,100 deaths. The country late Tuesday reported its biggest one-day death toll of 111.

The large-scale trial is being conducted in South Africa, Britain and Brazil.

My question is why was the first trial done in Africa when we have more cases in the US  (but then that would probably be done on an African American) and other European countries.