Robert Mueller’s investigation is over, but questions still abound. Not about collusion, Russian interference or obstruction of justice, but about the leading lights of journalism who managed to get the story so wrong, and for so long.

It wasn’t merely an error here or there. America’s blue-chip journalists botched the entire story, from its birth during the presidential campaign to its final breath Sunday—and they never stopped congratulating themselves for it. Last year the New York Times and Washington Post shared a Pulitzer...