Try these reasons for African dominance in the marathon.
1) Many (but by no means all) children lead a more natural, healthy life. Lots of traveling on their own two feet.
2) Running is away out of what in many cases would be a life in semi poverty; it's a way to earn money and buy land/provide for their families. It's a hard life- but no harder than working long hours at a physically demanding, low paid job.
3) They believe they can do it. So and so who lives just down the road has made a few dollars by running; why not them? They try- if they don't succeed the give up. "Running for fun" isn't really an African trait.
4) They don't confuse completing with competing. They see little value in finishing a marathon with a few thousand in front of them. It's make money or find something else to do.
5) All these things mean there are lots of potential fast runners training together. We only ever hear of the reasonably successful ones; not the ones that give up/are broken by the training load/simply discover they don't have the talent.
6) In the 1960s,70s,80s in the UK there were scores of clubs that had dozens of members training 80-120 miles a week just to make their club 6, 9 or 12 for a national championship race on road or country. hence the great depth we had at that time. Then the mass participation events came along and changed the nature of distance running. In some ways for the better but in terms of elite standards very much for the worse!