I can't say why it doesn't work only what I believe. Now this is a theory of mine so I'll try to explain the best I can. When a dog is in keep your training that dogs body for whatever is at the end of that keep. So if your train the dog to be under large amounts of pressure and stress. Swimming doesn't apply this type of work into a dog during it's keep. So yes 2hrs of swimming is hard. But let's take the amount of force( work) that it takes to swim for 2hrs,while it is hard, the dog only works a small amount for x amount of time to keep it's head above work while swimming. But in a show from the start force is applied on the dogs body in heavy amounts for x amount of time often shorter then 2hrs so the dog simply isn't worked for the show. Take strong man weightlifters, they lift a heavy weight for a short time. Do you think they walk into a gym and work out with small amounts of weights for longer reps? Or do they do heavy weights with few reps?