Originally Posted by Sy Reene

The degree to which disc brakes are better than rim brakes, is not important to the manufacturers. To the manufacturers, a large part of their impetus is that they needed to have something on the bike that was of a significant enough difference to make someone want to get a new bike. I suspect the bike makers were starting to realize that they'd soon hit a wall on just selling more gears. If you have a 6yr old rim bike, out of aero shaped carbon fiber, why would you want a new aero shaped carbon fiber rim-braked bike. And, not sure if by itself the disc brake appeal would have been successful, so hand-in-hand with disc brakes, came along the notion that you can get a more comfortable ride with wider tires, which disc brakes can accommodate.

Disc brakes have been a huge boon for the road bike industry as a whole. It has led to new wheel designs, enabled more tubeless tire viability which price higher than nonTL, sealant sale increases, brake pad sale increases, and creates more maintenance income for bike shops. And all of this together also adds up to reasons to get consumers to buy an upgraded bike. It's just business.