Here's an interesting thought.
Perhaps Epic just don't want to have it "officially" on the Wii U until the other two platforms are ready, not for the purposes of influencing Sony/MS, or anything like that, but to maximise number of licenses, and thus profits.
After all, until Sony/MS's new systems are released, Epic doesn't want to have developers migrate to UE4 for Wii U, because it will mean only a third of the number of licenses (because of no 360/PS3 versions), whereas if games for Wii U are also on 360/PS3, then they get the maximum number of licenses, and the way to achieve that is with UE3. They could theoretically port UE4 down to 360/PS3, as well, but that would take time and effort that wouldn't be worth it, since those systems will be deprecated soon.
By having Wii U support unconfirmed, but UE4-on-Wii-U third-party ports expected, they open the opportunity for unofficial UE4 on Wii U in the meantime if developers are that keen... and then, when developers have UE4 titles well into development for the other next-gen consoles because they're close to release, Epic can then open up Wii U support, and get more licenses out of them.
This also gives them the leeway they need to see how the Wii U performs before committing.