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.
I am just guessing a fairly small people asked about Tuskegee would know what it is but maybe it's just me. I am just suggesting maybe asking black people why they don't want a covid vaccine for example and address those issues.
That said, I recently read that most black people (falsely) believe that the Tuskegee subjects were deliberately infected with syphilis, but I can't find the source right now, and it's kind of splitting hairs anyway.
Phase 1 is essentially to test "will this drug cause terrible harm to people who receive it in a reasonably short time frame?" In terms of dosage guidelines, phase 1 studies often don't have variable dosing, or if they do, not an awful lot - because the question around optimal dosing is the target of phase 2.
(Sometimes you can roll 2 and 3 up together, for instance, if the difference in dosing is in duration of therapy rather than an actual difference in drug administered per unit time.)
Phase 1 usually has terribly small samples, because it's only looking for terribly large effects. Seeing homogeneity there, of any stripe, is neither unexpected nor problematic.
(A lot of the vaccine studies rolled up phase 1/2 together because we already have a pretty good understanding of what these delivery mechanisms do to people, and so while you'd be cautious in administering to the initial study participants, it really didn't merit a "test it on five people and hold your breath" approach.)
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.