Here's why:

* Mechanical waves require a medium: Mechanical waves, like sound waves, need a substance (like air, water, or a solid) to travel through. They are disturbances that propagate through a medium.

* Light doesn't need a medium: Light can travel through the vacuum of space, where there is no matter. This demonstrates that it does not need a medium to propagate.

Instead of a mechanical wave, light is an electromagnetic wave. This means it's a combination of oscillating electric and magnetic fields that travel at the speed of light.