shape of hatch back and estate type cars is such that they tend to have a low pressure area at rear due to air flow carateristics thus tends to "suck" water, dirt etc upwards onto screen. this is less of an issue with saloon "notch back" style cars as airflow is smoother over the shape. So it is less necessary on saloon cars. the other thing is that to package a wiper system into this area is not easy , geneally ther is little room between the boot lid and the rear screen and would no doubt cause styling issues.