Here's why:

* Composition: The crust is primarily composed of lighter elements like silicon, oxygen, aluminum, and sodium. The mantle, on the other hand, is primarily composed of silicate minerals with a significant amount of iron and magnesium. These elements are denser than the elements found in the crust.

* Pressure: The immense pressure deep within the Earth compresses the mantle, further increasing its density.

While the mantle does contain some lighter elements, the higher proportion of iron and magnesium makes it significantly denser than the crust.