Rear tire wear is normally much slower than front tire wear. This is because the majority of the weight of your vehicle is over the front wheels. This is also mostly because all of the steering is done by the front wheels. So they have more stresses to deal with than the rear wheels, which basically just follow the front wheels when turning is done. Also, most of the braking is done by the front wheels. Rear wheels that are wearing faster than front wheels will usually also show abnormal wear patterns, such as feathering, edge wear or cupping, indicating problems with the suspension of your car.
For more info, see 3 Causes of Tire Wear and Tear