Combination of several factors.

1. The country was bankrupt when Mandela took over - due to sanctions. It has never been able to get on its feet again, because when sanctions came, massive wealth was transferred out of South Africa.

2. As a consequence of sanctions, the government was unable to invest further in building education and housing in townships and homelands (which it used to do).

3. As a result of apartheid, very few people had been educated to the level required for the governance. Also, many people who would have been suitable had been killed by the apartheid government.

4. The people aren't very capable.