Why are there gold and diamonds in South Africa? Mainly because South Africa is big. 1.21 million square kilometers. Look at any chunk of land that big and you are almost certain to find valuable mineral deposits, which in South Africa's case include gold and diamonds. Add to that the fact that no one felt much need to respect the property rights of the people living on the mine sites and the ability to use those people as a cheap labour source, combined with European technical expertise, and you have a recipie for a low cost producer.

An interesting aside, an asteroid smacking into South Africa long ago aparently created conditions right for gold deposits to form. (Note the gold wasn't in the asteroid.)