It is simply a matter of cultural organization combined with genetics. White people lived in a cold climate where hunting and gathering was possible for only 5 months of the year. They had be frugal even when they were at the hunter-gatherer stage. This helped weed out the dumb and those unable to control the urge for self gratification. When agriculture was introduced, they were able to further increase their social development by virtue of being in a region perfectly suited for grain production enabling the development of even more complex societies and stronger states. Europeans have always been militarily advanced, even at the height of Islam's golden age they were able to colonize the holy land for 200 years. The development of strong states, based on grain production and of somewhat restrained and cowed populations allowed whites to then conquer the rest of the world. This does not mean white people were in any way exceptional because the Chinese had those same attributes and advantages even earlier than whites but did not pursue large scale colonization.
Africa where blacks live is tropical and has too many diseases to count. The stupid members of the population have not been weeded out to the same level as Europe because they could easily survive the outside temperature and subsist on hunting and gathering year round. This is why you still have hunter gatherers in Africa to this day but none in Europe even a 1000 years ago. Strong states were very rare in Africa because there was always more land than people. Whoever tried to create a strong state would soon lose his population as they fled taxation. As a matter of fact, the Bantu groups were still migrating and settling parts of Africa as late as the 1600s. Black people are lucky that the Americas exist, if it did not, the Europeans would have reach peak population at some point and colonized all of Africa as they did the Americas. South Africa would have only been the beginning, and as soon as they discovered Quinine, millions of peasants would have flooded in making the conquest of the Americas look like a picnic.
Tl;dr, White people are better organized and we are lucky they found the Americas more to their liking.