An Australian based company, SuperPro began manufacturing replacement bushings made from polyurethane after it was realized that OE rubber couldn't withstand the tough and extreme conditions found in the notorious outback. Rubber has been a traditional material for comfort, but is prone to vulnerabilities and deterioration. Polyurethane bushings ensure the suspension components will retain their desired geometry and last longer than rubber.
An added point we feel deserves mention, is that SuperPro tunes their bushings to work together as a set. Meaning each location may be stiffer or softer than another location on the same car. This ensures the car as a whole has the absolute best handling possible.