First spotted by Reddit user AuraWielder, Donkey Kong Country: Tropical Freeze appears to have been removed from the Wii U eShop, in both North America and Europe. This is most likely due to the fact the game is set to release on Switch later this week.
The Wii U version of Tropical Freeze was one of the Classics line of titles on Wii U, meaning that Nintendo was selling it at a reduced price as one of the must-have first party titles for the system. Speculation online is high that Nintendo may have removed it to avoid undercutting its upcoming full-price launch on Switch.
Whatever the reason, if you want to pick the game up cheap on Wii U then your best bet now is the second-hand section of your local gaming store.
[Update: It appears that Tropical Freeze may actually have been removed from the eShop on Wii U around February of this year, shortly after the announcement of the Switch port. It just wasn't noticed until now, probably because everyone's suddenly interested in Tropical Freeze again. Sneaky Nintendo!]