Why does the Ash Twin Project require rotation for artificial gravity?

Maybe the Ash Twin Project was started by the Nomai of the Ember Twin (since they were closer than the ones in Brittle Hollow), and I don't remember any platform of gravity in Ember Twin, they only knew the rotation method you mentioned and they applied it inside the Project.

I think the Nomai in Ember Twin didn't know about the gravity platform technology, the rotational method was the way to go, and since it was started with the rotational technology it wasn't interesting to change the mechanism at all since it was working that way. I'm sure they debated the idea.

Neither the gravity crystals nor the special flooring are capable of being toggled off and on without being physically destroyed (at least, nowhere else in the game do we see this capability).

A toggleable floor at this location makes sense from both lore perspective - the Nomai wanted an easy way to access the reactor - and from a gameplay perspective - to allow the reactor to be in the precise center around an "interesting" area, while still allowing easy access to it.

But honestly, I think the main reason is a game development desire to generate a mysterious backdrop for when you glimpse into the Ash Twin Project from outside, and having a moving background helps that mystery. For the same reason I think the core of Timber Hearth - and consequently, the ATP - was designed to have a star-like appearance, not only for uniqueness but for generating the backdrop.