Here's why:

* Mechanical Waves: These waves require a medium to travel. They are disturbances that propagate through a material, causing particles in the medium to oscillate. Examples include sound waves (traveling through air), water waves, and seismic waves (traveling through the Earth).

* Electromagnetic Waves: These waves do not require a medium to travel. They are disturbances in electric and magnetic fields that can propagate through a vacuum. Examples include light, radio waves, and X-rays.

In summary:

* Mechanical waves need a medium.

* Electromagnetic waves can travel through a vacuum (no medium).