Why haven't we had a woman president?

Less than a year ago, the Democratic presidential field featured a record number of female candidates, many of them with credentials as strong as any of the men in the race. As the primary carried on, however, the women in the contest gradually fell off one by one.

Elizabeth Warren, the last woman with a realistic shot at winning, dropped out last week after a poor showing on Super Tuesday. Her withdrawal ended the chance that the first woman president would be elected in 2020. Rep. Tulsi Gabbard is...