Here's why:

* Velocity vs. Time: When we graph velocity on the y-axis and time on the x-axis, constant velocity is represented by a horizontal line.

* Slope and Rate of Change: The slope of a line represents the rate of change between the two variables. In this case, the slope represents the change in velocity over time.

* Zero Change: Since velocity is constant, there is no change in velocity over time. This means the rate of change (slope) is zero.

In simpler terms: If something is moving at a constant speed in a constant direction, it's not speeding up or slowing down. A zero slope on a velocity-time graph reflects this lack of acceleration.