As @smoorman1024 stated, too much political instability in much of Africa. It's true that some countries are stable, but (e.g.) being next to an unstable country doesn't affect the risk in a positive direction.
Apparently things are changing. Compare Asia from 1975 and today. It doesn't take long for such a profund change. If I was an investor, I would think that now is the right time to start looking at this continent. Chiniese already do. Many africans are highly educated but lack opportunities to put this competence in application. This continent is full of unexploited opportunities.