Seth Masket:

[C]oncerns about electability become a self-fulfilling prophecy. Women and people of color may be just as interested in holding office as whites, but they are regularly told that doing so would be a long shot and would hurt their party’s chances, and so many of them (quite possibly the most qualified candidates) decline to run. Presumed to be unelectable, few are elected. And those who do run are often taken less seriously by political elites and observers, and thus given less support and media attention.