You have to remember that even at LHR SAA dropped the 747-400 as just too big for the route.
Manchester used to have SAA too, before they pulled out ,and even they have no direct scheduled link to South Africa.
Also South Africa is an incredibly expensive place to fly and visit. We planned to go this year but it was 4 times the price of 2 weeks in The Maldives
so a no brainer for us.
Nigel