Here's why:

* L waves are surface waves. They travel along the surface of the Earth, similar to how ripples travel on the surface of water.

* They are shear waves. This means they cause particles in the material to move horizontally, perpendicular to the direction the wave is traveling.

* Shear waves cannot propagate through liquids or gases. This is because liquids and gases cannot support shear stress (resistance to deformation).

In summary:

* L waves need a solid medium to travel.

* They cannot pass through the Earth's core, which is primarily liquid.

Let me know if you have any other questions about seismic waves!