Here's why:

* Uniform velocity means the object is moving at a constant speed in a constant direction. This implies that the body's position changes at a constant rate.

* Slope in this context refers to the rate of change of position (displacement) over time.

* Since the velocity is uniform, the displacement changes linearly with time. This means the graph of displacement vs. time would be a straight line.

* A straight line has a constant slope, and a straight horizontal line has a slope of zero.

Therefore, a body moving with uniform velocity has a slope of zero.