Here's why:

* Pressure: The lower mantle experiences significantly higher pressure due to the weight of the overlying layers. This pressure compresses the rock, making it denser.

* Composition: While the lower mantle is primarily composed of silicate minerals like the upper mantle, the minerals in the lower mantle are denser due to the pressure.

* Temperature: While the lower mantle is hotter than the upper mantle, the increase in density from pressure outweighs the effect of the higher temperature.

So, the lower mantle, with its higher pressure and denser minerals, has a greater density than the upper mantle.