Amazon routinely delays shipment of preorders, especially if you preordered early enough to benefit from the fluctuating sale prices on their website.
Expect to wait as much as a week beyond the release date of an item as punishment for preordering early.
One customer rep told me that the shipments are scheduled on a first-come, first-served basis, but this is clearly BS as you'll be able to order the item the day before it releases and ensure delivery the very next morning. You just have to cough up the full retail price and forego any discount you may have received for preordering early.
And for the record, your prime membership means nothing to them. It's advertised as a way to get fast shipping, but you can expect to be the last on amazon's priority list. You're the sucker they've already managed to reel in to spending 100 bucks on a worthless membership. They don't need to do anything more to impress you.