Question

why sliding friction is greater than rolling friction

Answer 1

Rolling friction is the resistance to motion experienced by a body when it rolls upon another. It is much less than sliding friction for same pair of bodies. When one body rolls upon another, there is theoretically no sliding or slip between them. And if both are perfectly rigid, there is no surface of contact.

Answer 2

Rolling friction must always be less than sliding friction, because in order for an object to roll, the force of friction between it and the surface must be large enough to keep the object from sliding.

Whenever one applies a sideways force trough the center of gravity of an object, that force has two components: 1) a direct force that tries to overcome friction and slide the object, and 2) a torque that uses friction to produce a rotation of the object by lifting its center of gravity over the leading edge.