Why do only downhill mountain bikers wear full face helmets? A lot of cyclists are concerned about safety and having the proper equipment, but I have never seen a cyclist riding in the city with a full-face helmet, nor have I seen a full-face helmet marketed for city/road riding. Is there a reason for this? 

I have a friend who broke several teeth after a tire got caught in some trolly tracks resulting in many thousands of $'s of dental bills, and know of another young woman who died in a similar incident after hitting her head on the curb. Both were wearing standard bike helmets. 

If the full-face helmet offers more protection to more of your head, why wouldn't some people want that, even if they're not doing some ridiculous jump on a downhill or BMX bike?



vs.
I think there will be a significant subjective element to answering this question, but pressing on:


Full face helmets are relatively heavy
Full face helmets offer less ventilation that standard helmets
Full face helmets are less aerodynamic that standard helmets
Riders don't want to look silly or weird
Full face helmets are not marketed to cross country MTB, road or casual cyclists.


However, given that a few decades ago cycle helmets were rarely worn, but are now common; it seems plausible that in the future lighter, more ventilated helmets will be developed and become generally acceptable for less extreme forms of riding.