Why is ground effect greater over smooth hard surfaces?
From a conservation of energy perspective, the rotor is transferring energy to the air, which then moves down and is deflected by the ground. A soft or drag-inducing surface will absorb more of that energy, whereas a smooth hard surface will deflect it.

Think of it like running: If you run on a hard surface, your down force creates an equal force up. If you run on sand, some of the down force is lost by moving the sand, and you are not propelled up/forward as much. This is why it is harder to run on sand than a solid surface.