Bill Morris has a theory:
In a marketplace glutted with too many titles – and in a culture that makes books more marginal by the day – publishers seem to think that if they just shout loudly enough, people will notice their products, then buy them. In other words, the run-on subtitle is literature’s equivalent of flop sweat, that stinky slime that coats the skin of every comedian, actor and novelist who has ever gotten ready to step in front of a live audience knowing, in the pit of his stomach, that he’s going to bomb.
But it's not true! As usual, it's the Internet's fault…a bookseller explains:
“I think it’s driven by Search – with a capital S – whether it’s Google or Amazon or whatever. A lot of our customers hear about books on NPR, and when they come in the store they can’t always remember the author or the title. The more words a customer might remember, the more keywords we can use to Google it. If a word is rather unique, we’re more likely to find it. With the river of books – with the river of everything – most people want to have more unique words associated with their product.”
Still, kudos to Morris for not only coming up with a theory, but testing it out.