Why do many stores have big after-Halloween sales on their Halloween merchandise?

Because after Halloween, they want to get rid of their Halloween merchandise. Once the demand for it is gone, the merchandise won't earn them any more money, and they need to free up room for Thanksgiving and Christmas. The same sale happens after most major holidays.

All stores try to provide their customers with everything they need. It is best to make profit if they know what they need and when. Since people buy a ton of items for Halloween, it is most common sense to prepare after-holiday items. Usually things to help clean up or just a parting gift after a holiday. This allows stores to make more profit on not just the holiday items, but after holiday purchases.

I hope this helps.

-szimbitskyy