IMHO, they just aren't needed for road conditions. Disc brakes work well in wet and muddy conditions and eliminate flexing that other types of braking systems have when used with wide tires. For road cycling, these really aren't problems, except for wet conditions and possibly mud. Properly tuned brake systems, pad design to some extent, and rider awareness of conditions can take care most if not all of those concerns on the road. CX and MTB is where disc brakes' strengths come in to give an advantage over other systems, and where they will probably stay.

advantages and disadvantages of disc brakes