If they never release at the same time as games on the other platforms and aren't supported with DLC then why would any customer pick up the DLC less late version that doesn't run as well? Those games didn't sell below expectations because they are mature but because Ubisoft specifically made the Wii U versions of less value due to their decisions.
If the games were late they missed out on marketing and hype. Everyone knows how much that means to so called mature titles which tend to be far more front loaded in sales. Launching that title after the others even at a discounted price it still has less percieved value.
The system is more powerful than 7th gen systems and still has issues running. Once again the game is of lesser value in comparison to other versions. No future support with additional content once again makes the Wii U version have less value in comparison to the others.
If Ubisoft can't treat Wii U customers as if they are as valuable as ps4/3 or X1/360 then they really shouldn't expect those levels of sales. Shitty performance late ports don't tend to sell on par with originals even on Playstation and Xbox platforms. I don't know why just showing up is supposed to be sufficient on Nintendo systems. These scenarios aren't something that Xbox and PlayStation owners would support on their platforms.