Here's why:

* Peridotite is a dense, dark-colored igneous rock composed primarily of olivine and pyroxene. It is the most common rock type in the Earth's mantle.

* Crust is the outermost layer of the Earth, and it is largely composed of lighter elements like silicon, aluminum, and oxygen.

* Peridotite is rarely found in the crust because it is heavier than the typical crustal rocks. It can be found in some locations where mantle material has been uplifted to the surface, such as in ophiolites.

Let me know if you have any other questions!