I figure the second biggest reason for books not getting noticed is because there are so many of them. We're over 40,000 for new books published every year, and some PR people and authors send out books to anyone and everyone--even inappropriate review venues.
(I still figure that the "biggest" reason for books not being reviewed is the lack of advertising support, as discussed in the preceding post.)
A tactic I use before a new book comes out is to contact everyone who might receive or want a copy for review. It's obvious, but some people don't use it. And, perhaps as evidenced by the preceding post, it doesn't always work--which may be a result of the "too many books" problem. Even though an editor or reviewer knows a book is on the way, she may be deluged by so many books (and accompanying announcements, email, etcetera) that the idea of the book is displaced from her thoughts.
At the same time, those operating in the more popular review venues (say, the New York Times) may rely on a rigid system or may have their review choices dictated to them.