Some companies don't want to devalue their products with cheap sales.
Its not worth the effort. WiiU production has stopped and the remaining unsold consoles are probably not that many.
Nintendo probably thinks that people who can not buy a WiiU now will buy Switch next year eventually.
People seem to be neglecting the retail side of the equation. Why would Best Buy, or any brick-and-mortar retailer, buy more of the new console when they have a bunch of old one sitting on the shelf not selling. They're doing the Retailers a favor by taking back the product and not sticking them with the bill.
This way the retailer feels OK about selling the new product. If the new products does not sell, there's some good faith that Nintendo will not screw over the brick-and-mortar Business with dead product. Along with that, Nintendo can keep the value artificially high for the Wii U and not confuse customers with multiple consoles for sale next year.