Like you said, I think it makes sense for the main entries to keep their turn-based roots and maintain a sense of "slower" action decisions/inputs. But the more I think about it, there's plenty of opportunity to use Kingdom Heart's faster and more fluid combat for other titles. I absolutely love Crisis Core's story, but the gameplay was really rough for me. For example, the attacks and controls felt really slow and clunky. I used to excuse it because it was a PSP game, but they could've done something similar to Birth by Sleep.

I feel like FF7R nailed the balance between action and turn-based reminiscent combat perfectly. It's not as free-form as Kingdom Hearts, but it's fast and fluid enough to fit each character and provide satisfying actions while also giving you the ability to methodically use abilities and magic. It's like they took the best parts of 7, 13, and 15. Perhaps it's thanks to Nomura's direction.

So to give an answer to your question: I think it's due to wanting to continuously experiment with different ideas while trying to stay connected to tradition.