Here's why:

* Angular velocity measures how fast an object is rotating or revolving around a central axis.

* Radians are a unit of angle measurement, where one radian is the angle subtended at the center of a circle by an arc equal in length to the radius of the circle.

* Seconds are the standard unit of time.

Therefore, angular velocity is expressed as the change in angle (in radians) over the change in time (in seconds).