Here's why:

* Waves require a medium: Waves are disturbances that travel through a medium, transferring energy but not matter. This medium can be a solid, liquid, or gas.

* Vacuum lacks a medium: A vacuum, by definition, is devoid of matter. There are no particles for the wave to disturb and travel through.

* Examples:

* Sound waves: Need air or another medium to travel.

* Water waves: Need water to propagate.

* Light waves: Are an exception, but they are not "true" waves in the classic sense.

Light waves as an exception:

While light doesn't need a medium to travel, it's not a traditional wave like sound or water waves. Light is an electromagnetic wave that consists of oscillating electric and magnetic fields. These fields can exist even in a vacuum and propagate at the speed of light.

Key takeaway:

Though light waves can travel through a vacuum, waves in general require a medium to propagate. This is why you can't hear sound in space, and why waves like sound or water waves can't travel through a vacuum.