To be able to switch Keyblades on the fly? The processing power would need to be powerful for that. It takes Kingdom Hearts 2.5 on the PS3 about 5 seconds to change your Keyblade after going through the menus. Also, in the event of boss battles, I feel like switching Keyblades is inappropriate. I've gotten about half-way through in a Critical Level 1 Run of KH2, and I've found that one of the best parts of it is when you find a strategy that works against a boss or enemy. The items do not make the player, but the skill does. I can understand why it'd be good to be able to switch weapons mid-boss, as it'd give you an option to switch up strategies and approaches, but I feel like the player should have to stick to one keyblade in a boss battle. Also, it'd be OP as **** if you could replenish/use items mid-battle. It'd make even Critical Mode a cakewalk.
Also, the reason that Kingdom Hearts 3 is on both PS4 and Xbox One is that they can both handle the Direct X Engine and Unreal Engine 4 (What KH3 is being made on), so it's logical, as they don't have to develop it for one console or the other, but can instead develop one game, and have it work on two systems. It's more money. A game for Wii U would need to be specially developed for that console, and considering the low install base (about 9.2 million v.s. about 18.5 million (PS4) + over 3.8 million (Xbox One) as of January this year), the return isn't worth it. There would be people who would buy Wii U's for a Kingdom Hearts game, (I bought a PSP for Birth by Sleep), but that would only boost sales by a small to medium margin, and not all fans would buy the game, especially if it was a spin-off, which chances are it would, as no mainline Kingdom Hearts title has ever been on a Nintendo Console (You could technically count the original Chain of Memories, but A, that was on the Gameboy, and B, Re:Chain of Memories was released on Playstation, and is considered the definitive and superior version by far). Kingdom Hearts on a Nintendo Console would be a great third-party title, but seeing as it hasn't been done in the 10+ years Nintendo has a series, and that Square Enix has no reason to put Kingdom Hearts on a Nintendo Console, it isn't going to happen.
Also, KH3 is the end of the Xehanort Saga, which started on Playstation. When I said it should stay on Playstation, it was more of a symbolic thing. It started on Playstation, let it end on Playstation, like it should. I'll speak for myself when I say that I wouldn't want any other controller but a Dualshock when I'm fighting Xehanort. Dualshocks The Dualshock 2 got me through the original Kingdom Hearts 2, the Dualshock 3 got me through Kingdom Hearts Final Mix and Kingdom Hearts II Final Mix, and I want Dualshock 4 to get me through Xehanort and Kingdom Hearts 3.