Here's why:

* Terminal velocity is the constant speed that a freely falling object eventually reaches when the force of air resistance equals the force of gravity.

* Acceleration is the rate of change of velocity.

* Since the object is no longer speeding up or slowing down at terminal velocity, its velocity is constant, and therefore its acceleration is zero.

Think of it like this: when you jump out of an airplane, you initially accelerate downwards due to gravity. As you fall faster, air resistance increases. Eventually, the air resistance becomes equal to your weight (force of gravity), and you stop accelerating. You continue falling at a constant speed, which is your terminal velocity.