Mountain bikes dropped the front derailleur years ago — why have road bikes not followed?

The answer is that road riding cares about the size of the steps between gears, and mountain biking largely does not.

On the road you are usually trying to hold a steady cadence at a steady effort for a long time, often in a group where the pace is set by somebody else. If the next gear is a 15% jump, you are either spinning too fast or grinding — there is no correct option, and you feel it for the next hour.

On a mountain bike the terrain changes constantly, the effort is intermittent, and you are choosing between roughly-right gears while picking a line. A big jump between ratios costs you very little.

So a single ring with twelve sprockets, which gives a wide range with large gaps, is a good trade off-road and a poor one on the road. Two chainrings let you keep small gaps and a wide range, because you get two overlapping sets of ratios rather than one sparse set.

One under-mentioned factor: a single ring puts the chain at a severe angle at both ends of the cassette, and severe cross-chaining costs efficiency and wear.

With two rings the chain spends most of its life reasonably straight, because you are expected to change ring as you move across the cassette. That is a real if unglamorous argument for keeping the front mechanism on a bike ridden for long distances.

The other half is that the things which pushed mountain bikes to a single ring do not apply on the road.

Chain retention. Off-road, front derailleurs drop chains over rough ground, and a dropped chain mid-descent is a real problem. Narrow-wide chainrings and clutch derailleurs fixed that, and removing the front mechanism entirely fixed it more.

Ground clearance and mud. A front derailleur is a mud trap sitting exactly where the frame is narrowest.

Suspension design. Removing the front derailleur freed frame designers from routing a mechanism around a moving pivot, which was genuinely constraining.

None of those matter on tarmac. A road front derailleur on a smooth surface is reliable, and there is no mud, no clearance issue and no suspension linkage to work around. So road riders were asked to give up gear spacing in exchange for solving problems they did not have.

It is worth saying that this is changing at the edges, and where it is changing tells you the trade clearly.

Single-ring setups are common on gravel bikes, where the terrain is rougher, chain retention matters again and riders accept bigger gaps. They appear on time trial bikes, where the range needed is narrow so a single ring can cover it with small steps. And they show up on commuter and utility bikes, where simplicity and low maintenance beat perfect cadence.

What has kept them off performance road bikes is that cassettes with twelve or thirteen sprockets still cannot deliver both the range and the spacing that two rings give. As sprocket counts keep rising that gap narrows, and the transition may well happen eventually — for the same reason it happened off-road, once the compromise stopped costing anything.