Originally Posted by bsyptak

It's not the speed necessarily though disc brakes are better in that regard too. It also saves your [insert $ value] wheelset from dying due to brake surface wear. Wheels conceivably last forever, well until the chain/cassette goes the way of the doodoo bird in favor of some sort of internal transmission. Wet riding doesn't gunk up the wheels & braking surfaces. Pads last longer.

Another reason is that manufacturers are always looking for ways to encourage/prod you to replace your bicycle with a shiny new one. The best way to do that is to make your current one 'obsolete'. Disc brakes don't work on rim brake equipped frames, though they could on the front with a hub/fork switchout. Who bought a new bike because their old one was steel or their old one had a quill stem or downtube shifters or a 5-6-7-8-9 speed drivetrain or center-pull brakes or single pivot brakes? Downtube shifters are over a half pound lighter than brifters. 10 speed is unnecessary.

Fast forward X years: marketing and forced obsolescence killed the rim brake