They might be waiting until the stock can be distributed throughout the different US Amazon facilities.
The first sale that went up was a bit of a fiasco, they had more buyers at the ready than they did stock in hand.
They are probably trying to get the distribution even out so that they won’t have a repeat of the UK sales experience.
And I understand exactly how frustrating it is to wait. I bought a Lightsaber from the Disney store, and kept checking every day to find out when it was shipped. I hate when I buy something from a company with 3 to 4 day delivery, and it takes them that long to bring it to UPS.