2016 Update
Disc brakes are now common on road bikes, even racing road bikes in the professional peloton. Most of the major bike manufacturers offer disc and non-disc options for most of their bike models. Some "endurance", "adventure", and "cyclocross" (drop bar bikes sold under "road" category, but intended for some off-road use) models only offer options with disc brakes now.
However, there is some opposition to their use in certain types of road racing. If a rider's leg contacts a disc rotor while the bike is going fast, that rotor will likely cause a serious injury (it's spinning fast and pretty thin, so cuts flesh easily).
The advantages of disc brakes are primarily in wet and/or muddy conditions. In dry conditions, a good rim brake can lock both wheels up just as well as a disc brake. So for road-racing under dry conditions, there's no real advantage to disc brakes, and appears to be some disadvantages.
Many of the "racer" style road bikes follow what's allowed for professional road bike racing, so it's possible that the current opposition to disc brakes in professional road racing will eventually lead to them becoming less popular on road bikes.
Personally, my next new bike will definitely have disc brakes. I don't race in a peloton, I do sometimes ride in the wet, and it's much cheaper to replace a rotor than to replace a rim or wheel. (rim brakes eventually wear down the rim and make you replace it)
Original answer
I think it mostly comes down to one primary thing: disc brakes weigh more and road bikes are supposed to be light. Also, you need a heavier wheel and heavier fork to handle the forces of disc braking, which compounds the weight.
Additionally, the advantages to disc brakes (working better in mud/dirt, easier to work with a suspension, work with really wide tires) generally don't apply to road bikes or aren't what the road bike was designed for. For any bike on the road the only likely advantage of disc brakes would be that they're more likely to work after you've ridden through something nasty (mud, puddle with oil on it, etc) that's deep enough to get on the rims, and you probably want to be avoiding any puddles you can't see the bottom of anyways.
Probably the final reason is simply that disc brakes (generally) cost more than rim brakes.
I have seen bikes made for the road that had disc brakes, but they're generally not the typical "go fast" road bike. (Example: Kona Sutra)
