That's pretty much it.
The 13 series, which went on longer than initially planned, took up development time from both Versus 13 (Which is now 15) and KH3. Once that was finally done and they were ready to work on both, the PS4 was already on it's way and Nomura had to step things up higher than previously planned for both games which were going to be on the PS3 originally. Later on, Nomura would leave FF15 to focus solely on KH3 and now they're still currently hiring more people to work specifically on KH3 at Square.
So yeah. That's why.
One thing and one thing only:
Well, that and Nomura's fetish for making outfits for Lightning.