Here's why:

* P-waves (Primary waves): These are compressional waves that can travel through solids, liquids, and gases. They slow down as they pass through the Earth's outer core, which is liquid.

* S-waves (Secondary waves): These are shear waves that can only travel through solids. They cannot travel through liquids or gases.

Therefore, S-waves completely disappear at the Earth's outer core, and P-waves are significantly slowed and altered. This creates a "shadow zone" where neither wave type is detected.