The main issue is that despite the power difference, the PS4 and Xbox One are pretty much twins so development on one can easily be done as a afterthought on the other. Especially in a situation like we have with the Xbox One, where it probably won't sell that well so people won't notice performance differences unless they are game breaking.
The Switch could probably be done but it will take additional resources, not only for scaling it back but adapting the various ways the game can be controlled.
Also for a troubled development project like Kingdom Hearts 3, they are probably doing all they can to just get the game out. Adding another platform would probably require a new development team and Nomura is said to be nuts when it comes to the details of the projects he works on leading to delays. KH3 on Switch is unlikely unless they change development leads like FFXV and someone re-works it from the ground up, plus the Switch completed dominates the Japanese gaming market.







