http://www.thebody.com/content/art54916.htmlWhile it's true that minority groups have been left out of clinical trials, there is also a lack of willingness to participate due to a mistrust with the medical community.
For Blacks and African-Americans, you can easily point to Tuskegee to understand why this mistrust exists. But you can go as far back as
http://www.medscape.com/viewarticle/479892_3">Dr. J. Marion Sims, the father of modern American gynecology, to get an even deeper sense of this mistrust.
Then there's the
http://www.ourbodiesourselves.org/book/companion.asp?id=31&compID=55">Puerto Rican Sterilization project, sanctioned by the US government, that dealt with population control.
These aren't conspiracy theories, but tragic abuses by our medical community on women and PoC. Fortunately, as a result, we now have informed consent, which was created as a direct result from these mistreatments.
The major challenge we now face is convincing these communities that they are protected from exploitation from the medical community. But because of these historical underpinnings, it's been difficult to get these communities involved.