Because it's cheap, easy to produce, and a relatively low maintenance material for a factory bushing. Which is not to say it provides the best performance, or that better materials can't be used to great success.
Also, most cars that do have a body that is rubber mounted to a frame still use rubber control arm bushings. Because it's cheap, easy to produce, relatively low maintenance, etc...
Poly bushings work great, and will outperform rubber bushings if you maintain them properly and pair them with other higher performance suspension components (like good shocks!)