Pundits and prognosticators are in a rare mea culpa moment, acknowledging how badly they misjudged Donald Trump as a presidential candidate and apologizing for being so narrow-minded. But they didn’t need to travel to dozens of campaign events or poll hundreds of Iowans to understand Trump’s appeal. All they had to do was understand one single number: $55,191. That was median household income, adjusted for inflation, in June 2015, the month Trump declared he was running for the Republican presidential nomination. That number is neither good nor bad on its own, but when you compare it with a second number,...