IMO the weather conditions. Education and academic research is a luxury, Europe for example has resources for survival in high abundance, on the other hand when much of your continent is arid and dry it places stress on animals and plants required by humans for survival. There was not enough time in the day to adequately dedicate to trying to find out new things when each day is a feat of survival. Africa is also huge, really huge, it doesn't show well on a map because they have to compensate for the earths curvature but google some comparison images and you'll realize how big it is. This prevented many African civilizations from interacting with other cultures as it was simply impractical to travel so far to a land so harsh to survive in. Additionally this creates population groups within the continent themselves developing independently with little contact with each other resulting in different (and sometimes opposing) cultural viewpoints and lifestyles.

As an example of the benefits of cultural exchange is the crusades which sparked the process that would later become the Renaissance. As ideas and knowledge from the middle east were attained and later on as the silk road was born, knowledge from the far east such as China and India followed back into Europe and vice versa. The advancement of a civilization is built upon its knowledge of the world, and knowledge is an accumulated asset adding in value as new ideas are discovered. Africa didn't have a whole lot of stuff the rest of the world wanted (except slaves) so they missed out on the cultural exchange fest going on in the major civilizations of the time.

Why does modern Africa have such a hard time catching up? When the rest of the world has been running at break neck speed for the past 400 years it seldom feels the need to slow down for the new guy to catch up. At the same time you can't give a seedling the same amount of water as an oak tree, it needs to grow and absorb a little bit at a time. Africa has been behind for so long it requires a certain period of time to absorb all these new ideas and developments. There's also a heap for political issues that have played a huge role but its still very much a continent in transition. Once again this is just my opinion based on my basic knowledge of economics.

TL;DR Because it lived so far away Africa wasn't invited to the cultural exchange party so didn't get all the gifts the other civilizations got.