A massive planet exerts a tidal force on a moon that causes the moon to obtain the same rotational period as its parent planet. A massive planet exerts a tidal force on a moon that causes the moon to align itself such that its tidal bulges always point toward and away from the planet. Most jovian moons were formed out of their planet's nebula with the same rotational period as their parent planet. Most jovian moons were formed out of their planet's nebula with the same orbital period. Ice is affected by tidal forces to a greater extent than rock.
Ice contains more radioactive elements than rock.
Ice has a lower melting point than rock. It is too large to have been formed in the jovian nebula that formed Neptune. It orbits Neptune in the opposite direction of Neptune's rotation. It has an atmosphere and a measurable greenhouse effect. It is colder than any other moon or planet.