FWD is better in slipper conditions for one major reason: the power is being applied to the wheels that are doing the steering.

To explain in a little more detail, if you are driving down a curvy road in the snow, there are obviously times that some of your 4 tires are going to lose traction. In a RWD car, the power is going to the rear wheels which are pointing straight at all times. So if your front tires lose traction, you will simply go straight off the road. In a FWD car, when you turn the front wheels to go around a curve, the power being applied is being pointed in the direction you want to go.

Of course if you have a good set of snow tires, then there is less chance of losing traction in your front wheels. But all things being equal, FWD is better in the snow.