A、Because their environment is becoming more and more dirty. B、Because the traditional ways of treating wastes are not efficient

Each day every person in the Untied States throws away more than five pounds of garbage. There is more garbage now than ever before and most of it is made up of the packages and cans in which we put our food. The traditional way of getting rid of solid wastes is quickly becoming inadequate. Many cities are experimenting with newer ways of handling their growing piles of garbage.
    One of these new ways is recycling. Through recycling usable materials are taken out of garbage and made into something else. These usable parts of garbage are put through the cycle of going from a raw material to finished product again.
    In some cities a machine called Hydrapulper is being used to help recycle garbage. It is like a huge mixing machine. The garbage is dropped onto a conveyor belt that feeds the machine; At the same time, water is pumped into the machine. With a mixing action, the Hydrapulper throws out the heavy metal objects that can later be sold as waste metal. The rest of the garbage—paper, food, plastic, rubber, glass, wood leaves, and other items— falls apart. The waste is then mixed with water and carried to another piece of equipment where glass, sand and small pieces of metal are thrown out.
    With the Hydrapulper, up to 95 percent of the original garbage is made again useful. The rest is turned into furnace ash.