part of the equation is that on drop bar, road bikes both the shifters and the brakes are normally out at the end of horns rather than mounted on the bars directly. That simply enables them to be more narrow. Then, one of the primary concerns with a road bike is wind resistance and being able to tuck the hand and arms inside the body helps to decrease wind resistance and increase speed -- the holy grail of road racing.

When comparing road bikes to mountain bikes everything comes down to one thing: SPEED. So, in road bikes there are a very few things that drive the design: aerodynamics, rolling resistance and weight. In the quest for speed, not much else matters...

On a road bike, a wide handle bar would push the arms outside of the body, increase wind resistance and slow down the bike.