Here's why:

* Direction: Angular acceleration describes the rate of change of angular velocity. Angular velocity itself is a vector quantity, with its direction defined by the axis of rotation (using the right-hand rule). Therefore, angular acceleration also has a direction associated with it. It specifies whether the rotation is speeding up or slowing down and in what direction (clockwise or counter-clockwise).

* Magnitude: The magnitude of angular acceleration is the rate at which the angular velocity changes over time.

In summary: Angular acceleration has both magnitude (how fast the angular velocity is changing) and direction (the axis around which the change is occurring). This makes it a vector quantity.