That's because when the sale started, TSW5 had not been released. Buy the TSW4 version, and it will be added to your collection.
I found the sale items by looking for TSW4 on steam and then looking at the dlc available for that product. Hope that helps.
Tsw5 is brand new and so are its dlc. Steam sale rules dictate that new games can not be on sale for a period of time. You will have to get the tsw4 versions and upgrade to tsw5
