I think the issue is that people sometimes are influenced by the idea of movie or song titles. They want something that sounds intriguing, but the typical reader needs more information to commit. The additional information winds up in the subtitle. "A Bridge Too Far" worked as a movie title, but interest in movies is helped along by the stars and things like reader recognition if a popular novel had the same title. To continue the example, the subtitle to "A Bridge Too Far" might be "One of the worst military miscalculations of WWII". In this example, the subtitle is infinitely better.