I agree that Africa is the birthplace of humanity, but what I want to know is why...?

The more advanced cultures in Africa slowly migrated north to Europe and Asia to find better land for farming and hunting and probably a more desirable climate. The tribes that migrated out of Africa began to change over time and their culture learned more as they continued to move into Europe and Asia. Over thousands of years these new cultures became more light skinned and evolved to match their climate and lanscape. The tribes that stayed in Africa did not evolve nearly as quickly and in some cases not at all.