First of all, it is not accurate to say that South Africa was granted independence in 1994. Instead, we should say that it was returned to native control in that year. This was the first year in which non-white South Africans were allowed to vote.
The reason that it took so long for this to happen was because South Africa had a much bigger white population than any other British colony. In other colonies, it was not much of a problem to hand power over to the natives because there was not a large white population that A) wanted to hold on to power and B) feared the black majority. In South Africa, by contrast, there was a large white population that had been oppressing the non-white population for decades. That white population resisted the idea of sharing power with non-whites. For this reason, South Africa remained white-controlled for much longer than any other British colony in Africa.