Is it really that surprising that not nearly as many African-descended people listen to Metal in general? Those in the Western world are subjugated to their own exclusive culture, whether they choose to follow it or not, and most likely have few avenues to explore that might actually lead to Metal. Those in the third world, like most others in the third world, work hard just to survive and don't have time for leisure and luxury. Free time breeds intellectualism and art. When you don't have that free time, you're most likely not going to find much interest in art when you're looking for food.
Of course, there is also the undeniable perception as Metal as the white man's music which surely turns some away as well, but that in no way makes the statement that Metal is for white people true.
And yes, this post is rife with generalizations. You kind of have to be general.