It appears Amazon UK has currently run out of pre-order stock for the Wii U version of The Legend of Zelda: Breath of the Wild. This could be a sign that Nintendo expects more sales will be for the Switch version.
If you click here, you can see that the listing on Amazon UK is no longer taking pre-orders for The Legend of Zelda: Breath of the Wild Wii U. Currently the stock is not available and there’s no date for when more units will be coming in. Amazon UK literally says “we don’t know when or if this item will be back in stock.”
You don’t have to worry if you live in North America as the US Amazon still has stock in for Zelda Wii U. Since the Wii U is on its dying days, many people are probably going to opt to get the Switch version instead.
Hopefully Amazon UK does get more stock for the Wii U version though. Not everyone has the money to buy a Switch at launch so those with a Wii U already just need to buy the game instead of a new console.