Here's why:

* P-waves are compressional waves, meaning they travel by compressing and expanding the material they pass through. This allows them to travel through both solids and liquids.

* S-waves (secondary waves) are shear waves, meaning they travel by shaking the material perpendicular to their direction of motion. They can only travel through solids. Since the outer core is liquid, S-waves cannot pass through it.

Let me know if you'd like to know more about seismic waves!