It's the exploit/explore tradeoff. In the cultural industries, most consumers (the majority of whom don't care to be more than that) prefer to exploit--that is, read or watch or listen to something very similar to what they already have--than explore.

We think readers should be a smarter crowd--and, thus, more upscale in their choices--than TV watchers and popular music listeners (as both groups comprise, roughly, the general population). This is possibly the case, but it seems to be cancelled out by the massive time investment involved in seriously reading a book: eight hours for a typical novel, as opposed to an hour for a TV show or 3 minutes for a pop song. The high cost of exploration means that the book world has the same problem and, unfortunately, the processes and people who are trusted to filter for quality, quite frankly, don't. Traditional publishers follow the market and "book buzz" is built from the opinions of highly influential non-readers.