It's still very difficult to convince black people to partake in medical trials, largely because their trust in the system was so thoroughly violated in the not-so-distant past: https://en.wikipedia.org/wiki/Tuskegee_Syphilis_Study

It's not hard to imagine a patient noping out of an experiment if he's the token black guy in a room full of white doctors poking and prodding at him.

I work for a company that makes medical devices for people with diabetes, (a disease which disproportionately affects people of color), and we go out of our way to ensure that we have a diverse cast of people who interact with the patients in our studies.