Ever since the N64 days, third parties have been pretty reluctant to put games on Nintendo consoles.
Main problems with the Wii U include the fact the hardware isn't up to snuff, the different achitecture of the Wii U compared to the PS4 and XB1, and the poor sales of the Wii U, doesn't allow third party titles to sell a whole lot.
With all these factors, it's really just a waste for most third parties to devote all this time just for third party Wii U titles that don't sell a whole lot.
"Just for comparison Uncharted 4 was 20x bigger than Splatoon 2. This shows the huge difference between Sony's first-party games and Nintendo's first-party games."