The reason is genetics and geography.
If you live on the continent of Africa, especially at altitude in east Africa, your genetics are advantageous in the sport of distance running. Your body has more red blood cells as a result of generations of your ancestors living at altitude
and you can achieve greater results in longer races, especially distances 5000m and up.
If you are caucasian, your ancestors moved away from the African continent to Europe and possibly North America. You are genetically less likely to excel at distance running as a result.
Your body also has had to carry more fat as a result of adapting to colder climates. You may be 5'10 and 150lbs which is very thing but you cannot compete against someone who is 5'6 and 120lbs because their body is fare more suitable to the marathon.
There are exceptions where the white man will come out on top of the African but they are not common at the highest level.