A discussion topic between writers will almost always turn to book reviews. It’s not easy to count on people who buy your book (or get it for free on a promotion day) to go back to the outlet that they bought it from and write about it when they’ve finished it.
This might be because they didn’t like it – an inevitability that every author has to face; not everyone thinks you’re as bright, witty, charming, creative, or as imaginative as you think you are – or it might be because it’s not terribly easy to post a review on many sites. It’s getting better, but it can still be daunting for readers; it takes time, involves writing something (and for readers who read on eReaders or tablets with clunky keyboard interfaces it’s a pain in the butt to write anything beyond “LOL” or “:-)”), and it’s inconvenient. When a reader finishes off a book just as they’re getting off the subway, there’s a good chance they’ll just not review it – even if they loved it and couldn’t put it down. They might mean to, but it’ll never happen; out of sight, out of mind.