Why is fixed better on wet?

OK, so a quick check with Sheldon shows that its not an original thought.

*The mountain biker particularly benefits, because the Biopace design somewhat smooths out the delivery of power to the rear wheel. In climbing on loose surfaces, the limiting factor is often traction. The rear wheel tends to break loose during the maximum power phase of the pedal stroke, wasting most of the cyclist's energy. The Biopace chainwheel works like a storage device, storing power during the main power phase of the stroke as the feet accelerate, then delivering the stored power to the rear wheel during the "dead center" phase when the cranks are near vertical. The same average amount of power is delivered to the rear wheel, but in a smoother, less pulsating flow. All the energy is used to propel the bike forward, without the high-power peaks spinning the rear tire or causing the bike to "wheelie.*"