SEO is one reason, but the reduced attention span of today's people surely is another. People just won't bother figuring out what a "mystery title" book is about.
I don't like the idea of attention span in the context of media consumption. It presupposes that the producer has some kind of right to everyone else's attention just because they made something. If you're not going to help filter yourself out from people who won't be interested in you, it will be and should be an automatic no.
