Here's why:

* Pressure: The lower mantle experiences immense pressure from the weight of the overlying layers (crust and upper mantle). This pressure compresses the material, making it denser.

* Composition: While the exact composition of the lower mantle is still being studied, it is believed to be composed primarily of silicate minerals like bridgmanite and ferropericlase. These minerals are denser than the minerals found in the upper mantle.

* Temperature: The lower mantle is also hotter than the upper mantle, but the effect of pressure on density is greater than the effect of temperature.

Therefore, the combination of immense pressure, dense minerals, and high temperature makes the lower mantle the densest region of the Earth's mantle.