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.

The Ash twin project is in the core of ash twin. It's just an idea, but maybe a new kind of artificial gravity was needed to counteract such a strong central force.

Edit:

I have an idea that seems more logical now. The ash twin is inside a core of rock that was designed to withstand a supernova. While it does manage to do this, it would get very hot inside the core. Perhaps the gravity crystal floors would melt under this temperature, and something less sophisticated was needed to create the gravity. The main flaw in this theory, is that I don't believe anyone was meant to be inside the core during the supernova, but the nomai were obsessed with science, and the molten chrystals could possibly damage or destroy one of the masks, which would be a terrible failure, or if something on the chrystals was released due to their melting it could float and damage something. Either way, if the crystals melted the Nomai would probably have gone with centrifugal gravity just because it would be "better" in a way.

The "conventional" way of generating some kind of "artificial gravity" (any kind of force that is perceived as pushing you down on the ground of some space station or whatever) in a science-fiction setting often is huge rotating rings, where the centrifugal force you would experience in a rotating reference frame pushes you away from the point the ring is rotating around, and thus pushing you on the ground of the ring-like structure.

For example in '2001: A Space Odyssey', there also is a station that consists of two huge rings.

I never played Outer Wilds myself, but this wiki article says that "The surface of the platform is made entirely of gravity pads, ensuring that the player does not fall off.". So that would mean, that it's actually the same technology the Nomai used elsewhere.

Which in turn means, I can't actually give you a game-specific answer, only the general, "out-of-universe" answer how a rotating ring would "create" something you would perceive as gravity when standing in the ring with your head pointing inwards and you feet pointing outwards.

The idea of a spinning ring acting as some kind of centrifuge is also illustrated really well in this video by the YouTube channel "Real Engineering".