Here's why:

* Displacement is the change in position of an object. It's a vector quantity, meaning it has both magnitude (how much it changed) and direction.

* Velocity is the rate of change of displacement. It's also a vector quantity.

If displacement is decreasing, it means the object is moving in the opposite direction of its initial movement. A negative velocity indicates movement in the opposite direction of the chosen positive direction.

Example:

Imagine a car moving to the right (positive direction). If the car's displacement is decreasing, it means it's slowing down and eventually moving to the left. This movement to the left would be represented by a negative velocity.