For some reason, strongman training has never really caught on with the black folk. I can only think of 2 black guys that have competed at the top(John Gamble, O.D. Wilson). Also strongman is a small and somewhat expensive sport, I doubt many urban youth get exposed to it, it isn't like powerlifting or bodybuilding where a kid can just pop into a gym and learn.
As to why some Europeans and Nordic types do better strongman than others is Popularity. Those countries have a strong bias toward sports that transfer over to strongman better than others, and strongman is actually a respected sport, not just a ESPN oddity, so you can draw from a bigger pool of atheles.