It is a limited edition box, 4000 pieces. They're divided among different outlets (US, EU etc).
If someone puts the box into their cart, it will remove one from the stock (so they dont have the case of two people ordering the last box at the same time), but if the person doesn't complete the purchase it will become available again. This may take some time.
And people also sometimes cancel it later.
And yes, there might also be a case of them only releasing a portion of the boxes for sale - but i doubt it.
If you didnt get it by now, you will risk not getting it. Its 4000 pieces after all!