Well, AC fans that own a Wii U buy them on other systems where the games don't lack contents or run at 25FPS.
I doubt that Wii U owners are more interested in Just Dance than Assassin's Creed, it's just more profitable for Ubisoft because JD games are cheap as hell to port.
JD4 came out when the franchise was still popular and most JD players are used to Wiimote controls. JD2014 on Wii U was bundled with the 8GB model and promoted on TV together with the Wii version.
There's an eShop sale of Ubisoft games at the moment and both ACIV and JD2014 are at 14.99 €.
ACIV is #1 in Germany right now and JD2014 isn't in the top 20... #2: Child of Light, #3: ZombiU, #4 Rayman Legends, #14: SC Blacklist, #15: ACIII, #16: Rabbids Land.
Well, the JD audience might not use the eShop that much...
EDIT: number of ratings on the German eShop as of the 21th of August:
ZombiU:4017
ACIII: 2854
ACIV: 2854
SC Blacklist: 708
JD4: 447
JD2014: 377
JD Kids 2014: 8 (lol)
