Irrigation probably presents the biggest barrier, especially in the Northern equatorial regions of Africa. Once you're utilizing 100% of the water available you can't grow anything else.
Sure you could get _an_ African farmer to American level yields, but that's different than what essentially is economic reorganization for the _majority_ of workers in African economies.
Also, remember this isn't an agile software project that can go through an iteration a day. You plant once a year, you harvest once a year; an entire continent doubling yields in only 15 iterations is pretty good.
Many areas of the world where yields are low suffer not only form lack of money and technology but property rights are likely as significant.
A buddy of mine actually has a start-up that focuses on that called Suyo: http://suyo.co/
the west is facing a population crunch, as ladies are spending their more fertile years studying.