Commons does, however, allow scans of 2D public domain works that have "scarecrow" copyright claims in light of Bridgeman v. Corel (which ruled that such scans/photos can have no copyright in the US due to a lack of original creative content).
Commons doesn't accept 2D photos of 3D works because these photos' lighting, positioning, etc. choices can give them original creative content. But a 3D scan isn't likely to contain such creative choices. It may be annoying to make one, but the US does not have "sweat of the brow" copyright.
It's a rare scenario not generally considered by PD-Art on Commons. Commons isn't actually that conservative (except in that they demand the work be free in both its country of origin and the US). Commons has very few 3D models in general though not for copyright reasons.
Commons doesn't accept 2D photos of 3D works because these photos' lighting, positioning, etc. choices can give them original creative content. But a 3D scan isn't likely to contain such creative choices. It may be annoying to make one, but the US does not have "sweat of the brow" copyright.