Here's why:

* The Mantle: Peridotite is the dominant rock type in the Earth's mantle, the layer that sits beneath the crust. The mantle makes up the majority of Earth's volume.

* High Pressure and Temperature: Peridotite forms under immense pressure and extremely high temperatures, conditions that exist deep within the Earth.

* Composition: Peridotite is mainly composed of olivine and pyroxene minerals, which are stable under these extreme conditions.

While peridotite is the most common rock type in the mantle, other rocks like eclogite can also form at extreme depths due to changes in pressure and temperature.

It's important to note that directly studying rocks from the deep mantle is incredibly difficult. We learn about their composition through indirect methods like studying volcanic rocks that originate from the mantle and seismic wave analysis.