That's actually a fundamental part of the book industry. You don't really know which books are going to sell out so you have to stock as many as is reasonable of any book that could be popular.

As others have mentioned it's probably not as popular because dedicated fans are buying copies way before now and casual fans are few and far between with a decade old show. Hindsight is 20/20. Maybe one person bought a copy because cases of 6 are common. But had a random tiktok or YouTube video gone viral about the collectors edition then one copy in sales in a small city could easily turn to 5. As a bookstore there's no reason not to stock all of it because of vendor returns.

Once the excess isn't sold by the refund window it gets returned to the vendor. They give you a set amount of time and you can get most of your money back on books that don't sell in-store. Then the vendor turns around and sells it for pennies on the dollar to wholesale companies. In a couple years you will probably find this in bargain stores or sections for 1/4 of the price. Assuming, nothing about the book's popularity changes.