Thing is, we have no idea how the game is and what it requires. What we saw at E3 is not what we will get. Now, for some (I think) logical points:
- The WiiU userbase is small, but it'll grow by the time the game comes out.
- Kingdom Hearts games have been on Nintendo Handhelds, so there is a market for it, where as the Xbox One has none, and even the ReMix versions aren't coming for it.
It doesn't really matter how the game is - no matter what, developing a Wii U port in conjunction with the PS4/One versions is going to take a pretty significant amount of time, effort, and money.
As for your two points...
1) It'll grow, sure, but by how much? Enough to justify the costs? Enough to make the venture worthwhile? At the rate it's selling at now, that doesn't appear to be the case, and while we could say that certain titles/events might cause a spike in sales, there's no certainty. Any Wii U port would be a gamble, essentially, and considering SE's recent financial woes, it's clear that they'd rather put their money in safer investments.
2) Just because a KH game has never appeared on an Xbox console doesn't mean that there's no market for it there. It just hasn't been catered to yet. I mean, going by that logic, there's no market for KH 3 on Wii U because a KH game has never appeared on a Nintendo home console.
As I said before, the PS4 and Xbox One are roughly comparable when it comes to hardware, which means it's much, much easier to develop between the two systems. Development of an Xbox One version is evidently cost effective enough to justify the risk.
Yeah, bullshit; like ShadowSoldier said, there is a fanbase for KH in Nintendo; releasing this on the U might bolster sales for it.
Why should they care? That's Nintendo's responsibility and Nintendo's alone. Square Enix has no responsibility, nor vested interest, in making sure the Wii U succeeds.
Square Enix's priority is to generate enough of a profit to keep itself afloat (as I said before, they're not doing so hot financially).
It's the same thing as saying releasing this on the PS4 might bolster sales for that device. I guess we're just used to getting shovelware on Nintendo...
SE has no interest in making sure the PS4 succeeds, either, so that's irrelevant.
This isn't a Nintendo exclusive phenemonen. If any gaming machine fails to sell, it's going to lose developer sort - whether it's made by Nintendo (like the Wii U) or Sony (The Vita) or anyone else (console history is littered with enough corpses to demonstrate this). If the PS4 or Xbox One fail to move units, they're going to be in the exact same boat.