Amazon, B&N, and others in the POD networks deal with investments in physical inventory. They use complex algorithms to help predict the optimal inventory position. It’s in Amazon’s best interest to show as many books in stock as they can. However, they also do not want to have an inventory full of books that do not sell. Until a book has a proven sales track record, the stores will be very conservative in their ordering.
To gauge reader demand, Amazon requires a minimum of 60 days of pre-sale status for all new titles. This allows them to calculate a two-week inventory forecast. Amazon's goal is to maintain two weeks worth of inventory of your title in their warehouse so they can display a stocking message of "In stock. Ships Today." They determine demand during the pre-sale period to build their forecast and order books from us as the pre-sale period comes to an end. If a customer orders a book during the pre-sale period, Amazon will send a message informing the customer that the book will ship as the pre-sale period comes to a close.