99% of the time, whoever does something the most will be the best at it. If people in western african countries are winning sprinting marathons more than the rest, that probably means they have more people running on a regular basis and training for it than the rest. I don't think one race has a biological sprinting advantage over others. Perhaps people in western africa, for whatever cultural or economic reason, are running more than anyone else. That could be an issue of geography or socioeconomics or a lot of complex, intricate things, but not race.