Stereotype, I'm thinking.
I was lucky because my HS (in NM) had a black guy who was all-State in xc and ran distance (well) in track, so I just figured that black guys could do any distance.
Then I was in college with Charlie Bevier, who holds/held? a course record at Van Cortlandt.
However, there's a lot of stereotyping (by coaches and self) that can keep a black guy from even trying the longer stuff.
Yes, the average American of West African ancestry probably has a higher percentage of fast-twitch than the average American of European ancestry. But those are just averages, not individuals. I think more coaches should be willing to try different tests (with all their athletes) that might reveal latent distance talent.