Releasing something on every platform can cause unneeded comprises in design and doesn't make me happy. I mean even recently we have had characters scrapped from Smash Bros because they would have been too hard to make work on the 3ds. This isn't me raging against the underpowered WiiU, I love the WiiU, it's seeing that they are targeting more powerful systems, using a powerful engine, and not wanting to see the game designed and held back by a different lowest common denominator. Now if their plan is to make the best game they can on UE4 without ever caring about what would or would not work on other systems, then let some other dev group take care of a port, that's fine. Got to also remember how far off 2 years is. Maybe UE4 will be working on WiiU by then, or maybe the next Nintendo system will be out. I would also rather them not make promises about a version of the game that may not even be relevant when the game is going to be released.
Also that wouldn't be a very safe bet. If it was a mobile exclusive I wouldn't even be in this thread/care about what the game is. As for PC, one of the last really great metroidvania's that I played was a pc exclusive (think it was later ported to tablets) so I wouldn't mind that either.