When coasting while roller coating, you eventually stop. why?

In real, physical machinery, there is always some friction.

Friction robs kinetic energy from the moving parts and dissipates

the energy as heat.

When kinetic energy decreases, that's just another way of saying that

the speed of the moving object decreases.

Eventually, when friction has stolen all of the kinetic energy, there is

no speed left, and the motion stops.

Your weight on the rollers creates friction, or drag. The drag eventually overcomes your forward momentum, causing you to stop.