When people vote for politicians based on moral appeals instead of practical qualifications, corruption is much more likely; politicians don't have to do a good job, they just have to appear to be on the "good" side.
Even the article notes that the first 15 years of ANC rule saw significant development, as does HDI data [0].
The issue was that Jacob Zuma essentially destroyed RSA thanks to his corruption. And the only reason he became President was because he headed the ANC's intelligence department from the 70s to the 90s, and as such had dirt on everyone.