Its a marketing gimmick to drive up the demand, methinks...
Even though I know that this is a marketing gimmick, I still fall for it.
When MAC had the brush set on their website, I didn't buy it because the case and brush handle looked cheap in bloggers' pictures.
Then, I found out that the brush set was sold out and I went frantic and kept stalking the site to see if they would restock.
When Nordstrom posted the collection on their website, I immediately placed an order for the brush set, but then they cancelled my order and said the brush set was unavailable, which is weird because they still allow you to add the brush set in your cart and checkout.