A car is drifting when the rear slip angle is greater than the front slip angle, This often doesn't happen in AWD and is widely viewed as not drifting
For RWDcars : With the power to the rear of the car you are able to use the front wheels for what they were actually intended to do....steer, and control the car, weird right? With the absence of power to the front wheels this allows flexibility for the drifter when deciding when he or she wishes to get on or off the gas at will. The counter balance between counter steering and throttle is more eminent in a RWD car allowing more control for the drifter, which in the case of AWD and FWD the Drifter is more or less fighting with the car and forcing the car to operate how it should rather than working with the car to achieve what we know as the art of drifting.