Why are Disc Brakes better in the Rain?
Discs have a much higher clamping force. For rim brakes, there's usually quite a bit of movement; you can run the brakes with 5mm clearance from the rim and it'll still work fine. With discs, the pads move a much smaller distance, allowing them to use higher leverage ratios. For example, people running hydraulic rim brakes found themselves crushing rims from all the power.
I imagine this allows disc brakes to have more authority while braking. Water isn't capable of forcing itself under the pads due to the high pressure. Discs also heat up more and can run more aggressive pad compositions (because rapidly wearing out an aluminum rim isn't an issue anymore).The best MTB brakes use sintered metallic pads, which still offer crazy power in the wet.