Because SE. Consider they are supporting Vita, you would ask why no KH1.5 & 2.5 on vita.
Because KH3 will not be on Vita?
You can play/watch the entire KH series on PlayStation home console now*. Well, two consoles, but one is the direct successor of the other.
As much as I'd love another BbS-style (or full game with Aqua; not just a "short episode") on Vita, it doesn't make much sense when they've been trying to put everything into one line instead of having everything scattered all over the place.
Even if they did put it, Vita owners that doesn't have PS4 will not be able to play the final chapter.
On retrospect, I am actually quite surprised that KH3 is on Xbox One due to how they're pretty much making everything for PlayStation home consoles, but I'm guessing it was some sort of failsafe in case PS4 bombed hard and Xbox One was doing well. I was even expecting a Xbox 360/Xbox One version of the compilation games before KH3's 2016 releases but that doesn't seem likely at this point.
*I'm not sure how the Browser/Phone games matter, but I'm guessing not much?
Especially 3. Why does SD3 get no love from Square?
By the GameDev Law #49225
"If the greatest game in a series never came to the west, thou shall not ever bring it back to life."
See:
Summon Night Craftsword Story Hajimari no Ishi
Valkyria Chronicles 3**
Of course, Seiken Densetsu 3
**Edy supporters can say what they want, but Alfons/Leila combi is GOAT. VC3 also has a Samurai! And a gramma with a grenade launcher.