Black athletes in the US are successful in every kind of sport - and that was long before the practice of doping became a commonplace. Track, football, basketball, boxing, tennis and so on - even golf. But we also see other racial and ethnic groups succeeding in US sports. We don't see that in E Africa, where it is only distance running - a doper's sport if there ever was one.
Athletes like Hassan and Obiri come from countries that have a doping culture in sports. That there is more testing in the US doesn't mean they will be prevented from doping - as we have seen from those few US athletes that have been caught. But wherever there is doping most aren't caught - with the recent glaring exception of Kenya, where doping clearly lacks a sophisticated basis amongst many athletes.
However, much of this distracts from my fundamental point which is not that athletes from Africa (and Jamaica) are successful - any country can produce top athletes - but their degree of success is so disproportionate it suggest they are almost a different species from everyone else. They aren't. But doping can make it look like that.
For those who think my views are somehow "racially" based, they aren't - I am also sceptical at the increased success of Norwegian athletes today.