New York Mayor Bill de Blasio’s withdrawal from the presidential race last week is a lost opportunity. A successful de Blasio effort in the Democratic primaries—his was anything but—could have sparked a necessary national debate over signs of rising disorder in American cities. We haven’t returned to the bad old days of the 1970s and 1980s. The troubles of that era had many causes: disappearing jobs, changes in social welfare policy, changes in cultural attitudes, deinstitutionalization of the mental health system, drug wars, crack cocaine, a surging crime rate—all intersecting to drain the vitality of neighborhoods and stamp countless urban...