It's a mind trick. You know they are only available for a short time. So do many other people. By making it known, they've created a sense of "urgency," which to the consumer means "buy it now or I might miss out!!!!!"
It maximizes their sales in a short period of time, and when they come available again, anyone that "missed out" has the opportunity to be part of the "in crowd."
Personally, I think it's a terrible business model.
It's similar to Mass Drop.
I think it makes it possible for a a small niche business to survive. It allows them to carry basically zero inventory, only print when they have volume & keep the price reasonable. Imagine if you owned this T shirt business. There is no possible way you could survive with the "standard" business model. The shirts just don't appeal to a large enough audience. Anything with large appeal will become available again.