Here's why:

* Rocky Composition: Mercury is primarily composed of iron and nickel, making up a large percentage of its core. It has a thin silicate crust and mantle.

* Lack of Atmosphere: Unlike planets like Earth or Venus, Mercury has a very thin exosphere, essentially no atmosphere. This means there's no wind or weathering to erode away its rocky surface.

* Cratered Surface: Mercury is heavily cratered, a result of countless impacts from asteroids and comets over billions of years. These craters showcase its rocky, unyielding nature.

While other planets like Mars and Venus also have rocky surfaces, Mercury's composition and lack of atmosphere make it the most "rock-dominated" planet in our solar system.