Here's why:

* Composition: Comets are primarily made of ice (water ice, along with frozen gases like carbon dioxide, methane, and ammonia) mixed with dust and rock.

* Elliptical Orbit: Comets have highly elliptical orbits, meaning they come very close to the Sun (perihelion) and then swing far out into the outer solar system (aphelion).

* Other Characteristics: Comets often develop a tail of gas and dust when they get close to the Sun, which is a result of the ice vaporizing.

While other icy objects exist in the solar system, like some moons and dwarf planets, comets are the most well-known for their icy composition and elliptical orbits.