Because they have more tread depth, so more effective in hydroplaning (thus help in maintaining better control)
If front tires are more worn out, the front tires will begin to hydroplane: losing traction before the rear tires, which will cause an Understeer. While if the rear tires are worn out before front tires, this will cause Oversteer.
Understeer are easier to manage by reducing the gas thrust, Oversteers are far more difficult to control
This should be true for both front and rear wheel drive. Always the best tires on rear (even for a front drive)
General rule of thumb is to keep your tires above the wear bar for proper water evacuation while driving in the rain. With that being said, having new, top tier tires (more research goes into the technology then cheap brands) with full tread depth will help evacuate water effectively, even during heavy downpour when hydroplanning can occur.
The misconception is that having good tires on the rear, may prevent the car from fish tailing and skidding off the road during hydro planning BUT the fact is that if you have worn out tires on the front and you hydroplane then you lose the ability to steer which is equally as important, if not more so then having traction at the rear in this situation.
If one is changing out a pair of worn tires with new ones and wants to know whether to put the new ones on the front of the car or the rear, I usually suggest having tires with the most tread depth on the wheels that put power to the road (FWD=front, RWD=rear).