It's an interesting question and one that has been asked before. NPR did a story in 2013 on this topic, but their question was a bit more focused than just "why are so many black people good runners?"
The observation that led to their story wasn't just that black people in general were over-represented among long-distance running medalists, but that Kenyans in particular were over-represented. Digging deeper, the story's investigators found that the best runners in Kenya also tended to come from the same tribal group: the Kalenjin.
I'm not going to repeat all the details in that story (which I encourage you to read), but the working answer that the investigators came up with is that there are both genetic traits and certain cultural practices that contribute to this tribe's success on the track. Unfortunately, from the point of view of someone who wants a concise answer, it is very difficult to separate and quantify the exact contributions that each genetic and cultural modification makes to the runners' successes.
Pubmed also has a number of peer-reviewed papers detailing the Kalenjin running phenomenon, but I could only find two with free full-access and neither had the promising title of "Analysis of the Kenyan distance-running phenomenon," for which you have to pay. Insert annoyed frowning face here.
I did a quick search of some Kenyan gold medalist runners in the 2016 Olympics and sure enough, several (though certainly not all) are Kalenjin. I'm less sure about the Ethiopian runners, since most research that I found online seems to focus on the Kenyans, but I'd feel safe hypothesizing that something similar can explain their dominance at the podium.
So, the short answer to your question is that it's not just "black people" who dominate the world of competitive long-distance running, but that very specific subsets of people (who, as it turns out, are black) do display a competitive advantage and that both genetics and culture account for much of this advantage.