Here's why:

* Wavelength applies to all types of waves: Wavelength is a fundamental property of waves, whether they are sound waves, light waves, or seismic waves like S waves. It represents the distance between two successive crests or troughs of a wave.

* S waves are transverse waves: S waves, or secondary waves, are transverse waves, meaning the particles of the medium vibrate perpendicular to the direction the wave travels. Think of a rope tied to a post, and you shake it up and down. The rope will move up and down, but the wave travels horizontally along the rope.

* Wavelength is measured along the direction of travel: The wavelength of an S wave is measured along the direction the wave is traveling, not along the direction the particles are moving.

In summary: While S waves are a type of wave, the distance along the direction the S wave travels is not its wavelength. Wavelength is the distance between two successive crests or troughs of the wave, measured along the direction the wave propagates.