Pro riders are often required to ride whatever equipment their sponsor requires. Many of those sponsors only sell prebuilt bikes with disc brakes, so they equip the pro riders with the same, to encourage the sale of more expensive frames, wheels and groupsets.
Many carbon rimmed disc wheelsets cost 3 to 10 times the price of quality aluminum rimmed wheels, yet weigh as much or more and have more spokes than a campy zonda wheel's 16/21. Some are sold with rims that have no deep profile to improve aerodynamics, so they are merely over priced wheels with no performance advantage.
With the current 6.8kg weight limit, most brands have a top of the line frame that can meet that weight, even with 4-500 grams of additional weight from disc brakes. If not, then the riders suffer a small weight disadvantage.