I ask this as I know other countries in Africa have more natural resources and oil etc, and I know some of the Northern African countries have been superpowers since ancient times but aren't now, so why is South Africa the most developed now?
This might be better suited to /r/asksocialscience.
Also, you might want to re-frame the question - by any metric of development I'm aware of (for example, HDI), South Africa isn't the most developed African country.