Why are young black American men such good sprinters? Culture, that's why.
For someone to be great at any physical activity, first they need to be a genetic outlier. You can work as hard as you want, if the body isn't there, you'll never be Usain Bolt. When an outlier, through chance or someone specifically identifying them, happens upon the sport that is right for their skill set, AND they have the work ethic required, you get a great athlete, a champion.
When a champion comes along in a culture, one who isn't just a great athlete but is charismatic and inspiring, it encourages a whole generation of young people to try sport. The more youth involved in a sport, the greater the chances of identifying others who are potentially gifted at that sport.
It is a self-fulfilling prophesy sort of thing. A great runner inspires others to run, helping identify and encourage the future great runners who will in turn inspire others to run.