I'd say that there are a couple factors at play. The first is that for years the audience for such a book was fairly small because the install base of CQ just wasn't large enough to make it financially viable. While that's probably not true anymore there is still a perception by many people that it is (especially among the people with expertise to write such a book).
In addition there continues to be an very high rate of technical change on the platform, so almost any book that is written will be if not out of date, at least incomplete by the time it's published, making the book less valuable.
Finally I'd say that even though the audience for such books has widened the demand for experienced AEM technical talent is still incredible high so most of the folks with the expertise to write such a book have too many other demands on their time.
