The bottom pulley has lost electrons to the inner surface of the rubber belt.
The positive ions remain on the pulley, but the transferred electrons are spread all over the inner surface of the rubber belt. So the pulley’s positive charge is quite a bit more concentrated than the negative charge of the belt passing over its surface.
The comb, attached to electrical ground, has a replaceable supply of electrons. These electrons feel the concentrated positive charge of the pulley and some jump across the gap.