Interview with HardcorGamer

Daniel:  The Wii U is a really hard call for us, but from our initial research the audience is not there yet, and I would be really psyched for there to be this public outcry of “Where is the Wii U? I will absolutely throw money at you and I wanna buy it and preorder it!” and all that.  And then we start working on it.

Nick Chester:  We’re seeing all these people flock, especially towards PS4, and people are abandoning their Wiis, and using their Xbox 360s as…  We don’t have a version of it because neither of the consoles are backwards compatible.

It's one thing to keep hearing that the demographics of the Wii U aren't aligned with regular third party AAA releases, but another to hear that a party game just doesn't have an audience there yet.

EDIT:
More info from a FAQ on the Harmonix forums

Q: Why not Wii / Wii U / PC?
A: Due to shifting production priorities and capabilities of the studio, we don’t currently have plans to release RB4 on the Wii / Wii U / PC simultaneously with Xbox One and PS4. While we genuinely appreciate the support of Wii Rock band users over the years, and the increased interest from PC gamers, the truth is that we had to prioritize opportunities on other platforms with historically larger RB audiences.

But I thought the Wii U was a casual console.

As I've said in every Rock Band 4 topic everywhere, if any 3rd party game should come to Wii U, its this one. Mistakes were made.

That's not a good sign to say the least.

So the Wii U doesn`t have a hardcore market AND a casual market? Then what market does it have then?!

please don`t tell me Nintendo only market.. TwT

Sounds like a weak excuse honestly. I wish they would just say "We don`t want to release a Wii U version because it`s not worth it. The install base is too small" Also, last I check, the Wii is retired, so of course people will move on from it. Most people do. : P

Well damn, because all my plastic instruments are for the Wii. :( I'm not really inclined to rebuy more.

AFAIK just dance 2015 sold more on the Wii than any other platform. Fair enough they skip the Wii U but are they skipping the Wii?  Because they shouldn't.  

Just Dance 2015 also sold more on the Wii U than the other platforms (aside from the Wii).

Rock band was a lot bigger on ps2, 360, And ps3.  The ratio is not the same as just dance.  

At this point, I'm not really surprised that almost, if not, all 3rd parties are skipping the Wii U.