I watched the documentory with Michael Johnson, very interesting.
It is not all down to slavery though. As others have said there are genetic differences, some African countries have a tendancy to have people with a particular body structure, as do Europeans, dutch people tend to be taller, and Italians tend to be shorter.
There are other differences too, if you do lung function tests you have to adjust for race. Generally black people have a larger lung capacity than white people, who have a higher lung capacity than asian people.
When did you last see an asian sprinter?
I think there is also something about being mixed race, you inherit things from both (all) sides. And descendents of slaves are usually mixed race because of the history of slavery.
So, in very general terms:
sprinters - decended from slaves
long distance runners - from Kenya and Ethiopia (obv very general as Mo Farrah showed earlier)
Running is a cheap sport, for all sports that require equipment you have to take in factors such as income and availability of the equipment in particular countries.
Ethiopia has a single skier at the winter Olympics, he learned to ski in the US and had to form a skiing organisation. In some European countries skiing is just something everyone does, so there will always be more European and Canadian skiers than African, nothing to do with race but to do with the availability of equipment and lack of snow.