Basically because the aerodynamics.
An sedan vehicle make the rear wiper unnecessary because the car have an better airflow around it, this make the airflow to "clean" the rear window from water and debris at high speeds, but hatchbacks, SUVs and wagon vehicles have the rear wiper because of their shapes that produce behind them a big turbulent zone. This turbulence zone avoid the airflow to "clean" the rear window, and can even lift some debris (like dust and small stones) from the ground when the car is passing by.
Some sedans have an rear wiper like some versions of the Mitsubishi Evolution (like the one in this photo http://img150.imageshack.us/my.php?image... ), but it isn't very usual to see this kind of thing.