Here's why:

* Longitudinal waves are waves where the particles of the medium vibrate parallel to the direction the wave travels. Think of a slinky being pushed and pulled, the coils move back and forth in the same direction the wave travels.

* P waves (Primary waves) are compressional waves that travel through solids, liquids, and gases. The particles in the medium are compressed and expanded as the wave passes, moving in the same direction as the wave.

In contrast, the other body wave, the S wave (Secondary wave), is a transverse wave. In transverse waves, the particles of the medium vibrate perpendicular to the direction the wave travels.