Here's why:

* S-waves are shear waves, meaning they cause particles to move perpendicular to the direction of wave travel. Think of a rope: if you shake it up and down, the wave travels horizontally, but the rope itself moves vertically.

* The Earth's core is primarily liquid. Liquids cannot support shear forces, meaning S-waves cannot propagate through them.

* P-waves (primary waves), on the other hand, are compressional waves, meaning they cause particles to move in the same direction as the wave travels. Like a sound wave, they compress and expand the medium. Since liquids can be compressed, P-waves can travel through the core.

Therefore, S-waves are reflected back at the core-mantle boundary, while P-waves continue their journey through the core. This difference in behavior helps seismologists understand the Earth's internal structure.