Water molecules have a negatively charged and a positively charged side. Since opposite charges on adjacent molecules attract each other, the molecules cling very tightly together. Wax molecules on the other hand, are neutral, so when rain falls on the body of wax-polished car, the water molecules are pulled more strongly towards each other than they are towards the non-charged wax. The water starts beading up rather (photo, below) than spreading out over the surface of the car.
However, the car that is not polished with wax can have lots of charged molecules sticking out. These attract the water molecules, drawing them away from each other and onto the surface. Eventually water does not bead up.