Here's why:

* Composition: Unlike the gas giants Jupiter and Saturn, which are primarily composed of hydrogen and helium, Neptune is mainly composed of heavier elements, including water, methane, and ammonia. These elements exist as ices under the extreme pressure and temperatures found within Neptune's interior.

* Density: Ice giants are denser than gas giants. Neptune has a density of 1.64 g/cm³, which is significantly higher than Jupiter's density of 1.33 g/cm³.

* Internal Structure: Neptune has a rocky core surrounded by a layer of ice, followed by a thick atmosphere of hydrogen, helium, and methane. This structure contrasts with the gas giants' mostly fluid interiors.

While Neptune does have a thick atmosphere, its abundance of heavier elements, especially ices, distinguishes it as an ice giant.