Here's why:

* Acceleration is a change in velocity. Velocity has two components: speed and direction.

* In circular motion, the speed of the object may be constant, but its direction is constantly changing. As the object moves in a circle, it is constantly changing direction towards the center of the circle.

* Any change in velocity, even if only in direction, constitutes acceleration. This acceleration is called centripetal acceleration and is directed towards the center of the circle.

Therefore, even if the object's speed is constant, its changing direction means it's constantly accelerating.