Here's why:

* Light as an Electromagnetic Wave: Light is an electromagnetic wave, meaning it's made up of oscillating electric and magnetic fields. These fields interact with the charged particles (electrons) in a material.

* Interactions Slow Light Down: When light travels through a medium other than a vacuum, it interacts with the atoms and molecules of that medium. These interactions cause the light to slow down. The denser the medium, the more interactions, and the slower the light travels.

* Vacuum: No Interactions: In a vacuum, there are no atoms or molecules to interact with. Light travels unimpeded, reaching its maximum speed.

The speed of light in a vacuum is a fundamental constant denoted by 'c' and is approximately 299,792,458 meters per second.