After opening to middling reviews, “The Greatest Showman” has become one of the surprise hits of the season and no one seems to know why. On one level, it is a chronicle of the rise of P.T. Barnum from a young unemployed man to creator of what Americans remember as “The Greatest Show on Earth.” On another, it tells the tale of the circus that toured the country by train for spectators who could never get to New York. Why do audiences love the movie after so many critics dismissed it?