The Wii U Pro Controller is not compatible with the Wii Accessory Port. As for production and provision, the Wii Accessory Ports and Wii Classic Controllers were kept in production for the purposes of testing, but only for testing, and continued to be produced and distributed well into the Nintendo Switch lifespan as part of the SDEV Development Kit.
There aren't random warehouses full of the stuff, they fulfilled a purpose. Before Nintendo Switch launched, Nintendo did not have an official controller that used USB. Now they do, so there's no reason, economic or technical, to stick with the Wii Classic Controller Pro, over the Nintendo Switch Pro Controller, and a bunch of benefits to the Nintendo Switch Pro Controller, like its motion controls, NFC, etc. devs might want to use or test.