When used for a long time, slippers with rubber soles become slippery because:

⇒ For a given pair of surfaces, friction depends upon the smoothness of those surfaces.

⇒ Due to continuous rubbing of slippers with the ground, the rubber soles(surface) of slippers becomes smooth.

⇒ As we know that friction decreases when a surface is smooth.

⇒ As a result, friction decreases between rubber sole and the ground.

⇒ Thus, slippers become slippery.