I don't know if it's the cost, I've done the CT 4 for $200 before. Maybe it's just that 5 shirts is a significant bolus to most people's wardrobes. 3 at a time, or maybe 4, seems reasonable per quarter what with normal wear and tear. When you need five to tip a discount you're motivated to wait for more styles you really like to be released, or you get stuff you don't want. Then stuff runs out in your size. Better to just buy a few full price. Oh wait, hey maybe this is a great business idea!
Edit: Also I do believe this is just a one week promotion. It's a really good way for people to refresh their wardrobe if they can take advantage of it.
This is a short term promo. We've run it in tbe past a couple of years ago with great success. It will go back to the 3 shirt bundle once this ends.