I think the reason European sales are so low compared to the US is the lack of compelling games. The Wii U's biggest games are Nintendo Land and New Super Mario Bros U. Nintendo Land is a new IP, so there was no history of its performance in Europe. But looking at sales in Europe in relation to US, it doesn't look like Europe really loved it.
As for New Super Mario Bros U, Europe doesn't love 2D Mario quite as much as the US - NSMBWii European sales were less than half of what it sold in North America. Europe simply isn't as excited for 2D Mario as other nations. With a game like Mario Kart, Europe seems to match NA - Mario Kart European sales were ~85% of what it sold in North America, much bigger than with NSMBWii. This makse sense given that Europeans seem to love racers so much.
The Wii U also lacks much of what made the Wii attractive imo. I don't think tablets are even remotely as close to as exciting as motion controls. Sure, the Wii has motion controls, but it's no longer new. I think the Wii U is going to lack that instant appeal that the Wii had.