There are several reasons why some civilizations grow more powerful than their neighbors and eventually abosorb them.
The natural environment plays a huge role in determining the greatness of a civilization. The ancient Egyptians settled near the Nile River Delta where the annual flooding resulted in tremendously rich soil. This allowed them to produce a surplus of food so they could concentrate on other pursuits such as architecture and military training. Great Britain is an island which was able to rely on the English Channel for defense. This afforded them protection from other enemies who might have invaded them more often. The Kingdom of Mali rose in the center of very rich salt mines which allowed it to develope a strong economy.
Cultural factors also play a role. The Aztec Empire followed religious beliefs that served to turn them into imperialists. They believed that a steady stream of sacrafice was needed to keep their world intact, and therefore they were pushed to become a militarisitic power in their region.
Both of these factors help explain why some civilizations are able to take over their less fortunate neighbors.