Here's why:
* Centripetal acceleration is always directed towards the center of the circular path the object is following.
* Velocity has both magnitude (speed) and direction.
* Since centripetal acceleration only changes the direction of the velocity (and not its magnitude), the object's speed remains constant while its direction continuously changes.
Think of a car going around a circular track at a constant speed. The car's speed doesn't change, but its direction is constantly changing. This change in direction is caused by the centripetal acceleration.
