Here's why:

* Earthquakes originate in the crust: Earthquakes are caused by the movement of tectonic plates, which are pieces of the Earth's crust.

* Crust is brittle: The crust is the outermost and most brittle layer of the Earth. This brittleness allows it to fracture and shift, causing earthquakes.

* Seismic waves travel through the crust: The energy released by an earthquake travels as seismic waves, which are felt most strongly in the crust.

While the other layers of the Earth are also affected by seismic waves, the effects are less dramatic than in the crust. For example, the mantle is more viscous and can deform rather than fracture.