The problem with democracy in certain African counties is that people still have no choice in there. They vote for the most democratic president, but the elections are set up and the nation ends up with dictators, and it's been going on and on for years. @Gyfts is right, corruption isn't something that can be easily broken, it can take decades and many revolutions
Why it's so much more difficult in Africa?
Honestly, it's a rather difficult question, but I can't think of a better explanation than simply geographical isolation from the West. It is well-known from the world's history that even thousands of years ago when people used to live in tribes the location was crucial. Often those closely situated could find a common language easier and form bigger groups together (which later became civilizations), as opposed to with those that were located far away and were always considered hostile.
See, when the democracy started to spread in the West, Africa was kind of left out, while other countries formed unions, arranged meetings, and discussed regulations, future directions. Maybe that is one of the reasons this system wasn't fully and accurately implemented in African countries.
In addition, things are complicated with the fact that many countries in this continent don't want to become democratic, many still run autocratic regimes, and this surrounding also has an impact and slows the movement in the democratic direction for other countries.