The UCI has banned all cyclists from using disc brakes, after being asked by the AIGCP (Association of Independent Professional Cycling Teams) and Cyclistes Professionnels Association (CPA). In 2013, Chris Froome was the first Tour de France winner to say that he dislikes disc brakes. The CPA wrote a letter to the UCI asking them to ban the use of disc brakes on the pro peloton.
There are a few obvious reasons. The brakes provide more stopping power than rim brakes and are easier to adjust for different circumstances. These features make them the best choice for racing and steep descents. And, the fact that discs generate less force than rim brakes makes them the best choice for many cycling enthusiasts. But, despite the cost, the benefits far outweigh the risks. The discs can be more expensive than conventional braking systems, so they're not for everyone.