Discs are much heavier when you add up the disc, beefier hub, addtional spokes, heavy caliper, and additional material on the rear triangle and fork to withstand the stress.
They're better for wet weather but rim brakes are not that bad in the wet, and work just fine when it's dry.

Discs have recently been made legal for UCI cyclocross. In CX they can be an advantage in mud. Supposedly Shimano has a hydraulic road/cross brake/shifter in the works. If a lot of manufacturers get behind road/cross discs then they'll get lighter and better. But the regular rim brakes will still be lighter and work well enough.