Many parts of Africa are not politically stable enough to support this manufacturing. You should watch this Planet Money article to understand why Manufacturing (of T-Shirts in this case) move from one country to the next.
Probably true. I wonder why nobody ever counts Africa in these discussions. It's always "but what about when all the poor Asian workers start to earn higher wages and there are no more cheap countries left?".
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.
http://apps.npr.org/tshirt/#/title