>>1
Genetics. The race is just slightly less smart in average. Small variations in IQ can have dramatic impact on a large scale country. See how they had as much time as the rest to develop, and they often had much better natural conditions (favourable weather, awesome natural resources, oil, diamonds, food grows on trees faster than you can rip a fart, etc.), and they managed to fail every single country they control. If they had been smarter, they would have enslaved the whites and taken control of the world, or at least they would have prevented the whites from slaving them by kicking their asses through strategy and wise usage of their superior resources, but they didn't.
They also live in nigger cultures (note that nigger is a culture, not a race) which value crime over education, and are poor, indeed, but these features, which help their stupidity, are also a consequence of their stupidity.