I have no insight into how volunteers were recruited in the study but I would imagine that it is partially a cycle of African Americans being less likely to volunteer due to a history of mistreatment by medical community.

A lot of studies also recruit people who have a connection to the study (maybe friends/family members, students at the local university, etc...). If there is less diversity in the company and people running the study, I think this would also lead to less diversity in the participants. Again, I have no knowledge of how this one was done.