Here's why:

* Pressure: The immense pressure from the weight of the Earth's crust and the layers above it increases significantly as you go deeper into the mantle. This pressure compresses the mantle rock, making it denser.

* Composition: While the mantle is primarily composed of silicate minerals, the composition can vary slightly with depth. Some minerals are denser than others, and their distribution can contribute to density differences.

* Temperature: While the mantle is hot, the temperature gradient is not as steep as it is in the outer core. However, the temperature does increase with depth, and this can play a minor role in density changes.

Therefore, the bottom of the mantle, where the pressure is greatest, is also the densest region of the mantle.