Here's why:

* Acceleration is the rate of change of velocity.

* Velocity is speed in a specific direction.

* If the speed is constant and the direction doesn't change, there's no change in velocity, and therefore no acceleration.

Think of it like this: Imagine driving on a perfectly straight, empty highway at 100 mph. If you keep the speedometer needle steady, you're not speeding up or slowing down, meaning your acceleration is zero.