many reason :
1. it's UK where Wii U isn't that popular
2.It's third party games where most of people bought Wii U for their first party games
3.It's a current gen game that utilize last gen graphic (on Wii U)
4. The game heavily marketed for PS4 and the PS4 version released 6 month before Wii U version.
5. comparing the copy price, PS4 version it's more cheaper then the Wii U version in some store (even some have bundled on it)
6. Let say Ubisoft it's very kind to support the Wii U version even they knew it it will gonna happen like this.