It seems to me that if all of the items went up at the same time there would be fewer single item orders from the same people (because people can just place a single multiple item order). If they're behind in processing orders this could be an important consideration.
People would also be more likely to order multiple items, in general, I think. If you buy something on Monday and then another item you like becomes available Thursday you're less likely to buy it than you would have been if you were just able to fill your cart all at once.
Good point!!