Here's why:

* Constant Speed: Light travels at a constant speed in a vacuum, which is approximately 299,792,458 meters per second (approximately 186,282 miles per second). This speed is denoted by the letter 'c'.

* Special Relativity: Einstein's theory of special relativity dictates that the speed of light in a vacuum is a fundamental constant and is the same for all observers, regardless of their own motion.

* No Mass: Light has no mass. Acceleration requires a force, and force is proportional to mass (Newton's second law: F=ma). Since light has no mass, it cannot be accelerated.

However, the speed of light can change:

* Mediums: When light travels through a medium (like water or glass), it slows down. This is because light interacts with the particles in the medium, causing it to change direction and speed.

* Gravitational Fields: Light can be bent by strong gravitational fields. This bending isn't technically acceleration, but it does change the direction of light.

In summary: While light doesn't accelerate in the traditional sense, its speed can change due to interactions with mediums or the influence of gravity.