In order for research to be most effective, the people included need to be as diverse as possible. That is why the hundreds of papers each year that are surveys of psychology undergraduates who got extra credit come up with the kind of crazy conclusions mainstream media love to write about, but don't have the credibility of clinical trials.
In America, diversity in research is a struggle. Black and female patients are less likely to agree to participate in research, despite being offered more frequent opportunities to participate.