I found this interesting blog post by Dean Wesley Smith:
http://www.deanwesleysmith.com/?p=1742
His premise is that the big publishing houses treat new books as perishable produce (with the required press to "sell now" before expiration) - while the reader knows books are not perishable and want to get the book at their leisure ... or when
they want it.
Just a very good discussion on publishing.
Marilyn