The stopping distance on a bicycle doesn’t depend on the break performance as much as the amount of traction available. Breaking too hard will simply lock the wheels, especially in the wet. Disc breaks were introduced for bmx bikes due to improved performance in the mud. On road bikes disc brakes are overkill, add weight, are more difficult during wheel changes and are more complicated to maintain not to mention that the bike frame needs to be modified for them. All in all much more expensive.