Because giving a medicine, untested on humans, to an African during this time of panic and mistrust(justified, to be sure) is a ridiculous risk.

Role a dice to see if you can save Dr.Khan.

- If it works[<50%], awesome he lives and they get to work on making more of the stuff while trying to contain it.

- If doesn't work[>50%], the already deep mistrust and panic of the people will increase and everyone freaks out. If later they actually do find a cure, who's going to believe them?

- Third option. Test it on someone who isn't African so whatever the results are, you don't have a PR disaster & mass-panic on your hands.