Thats actually a genuinely good and interesting question !?
I wonder is it as people can identify in some way with the experiences and narrative in a book that draws them to it, or is it the exact opposite perhaps that its the fact that it's something they have NO knowledge of experience of which makes it fresh and interesting for them to read - or like most things in life somewhere in the middle of those two things perhaps. But it is a good question and I suppose if it can be answered them a publisher could make a lot of money pumping out what we all want to read !?