Amazon, B&N and others in the POD networks are dealing with investments in physical inventory. They use complicated algorithms to help predict the optimal inventory position. It’s in Amazon’s best interests to show as many books in stock as they can. But they also do not want to be stuck with 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 28 days in 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 use demand during the presale to build this forecast and then they order the books from us towards the end of the presale. You'll notice you can buy the book today from Amazon and when you do you get a message letting you know that you will receive the book at the close of the presale.