Basically there are a lot of reasons, but it all comes down to a small aerodynamic efficiency gain is no where near the sacrifice required in the other discipli

Basically there are a lot of reasons, but it all comes down to a small aerodynamic efficiency gain is no where near the sacrifice required in the other disciplines. in other cycling disciplines the efficiency of a rider on a flat smooth road, in the wind (no drafting) without significant corners or descents (no technicality) matters far less, and/or how you get efficiency is different. In fact in other disciplines that situation basically doesn’t exist. Keep in mind that for a lot of people, for a long time, the pinnacle of triathlon, and primarily how these bikes are marketed and designed, is around the Ironman World Championships in Kona. The 112 mile course only has 4500ft of elevation gain. That is very flat. Additionally some top triathlon pros consider even themselves to be basically beginner level bike handlers compared to road bikers. Which is basically beginner level bike handling compared to mountain biking. Why can you win races and compete for world championships if you have such poor skills? Because in that discipline courses are designed such that one needs virtually no skill, and the bikes are thus not designed at all for handling. Meanwhile in road bike racing, such as the Tour de France, riders can win races or stages due to their descending skills despite bike handling not being a top priority of the discipline (look at Tom Pidcocks performance vs Chris Froome on Alpe d’Huez in 2022). Also in road bike racing, protected riders are protected from the wind 99% of the time or more. It’s more important to be able to get close to the rider in front of you, and to still have control of your bike, than to be extremely aerodynamic yourself as most of those gains are lost in the draft. Especially true in the peloton, where riders frequently talk about the feeling of being “sucked along” and not having to pedal basically at all. It’s almost hard to believe, but it’s wild when you actually experience it. In mountain biking for instance, control, grip, flexibility of body position etc is much more important than just outright aerodynamic efficiency. Rolling resistance too is a much bigger proportion of the energy lost than on smooth roads. Speeds are also naturally slower due to the fact that courses are not only off road, but often much steeper than what is typically found on road. Additionally aerodynamics matter little if you can’t complete the course without crashing/blowing up your bike. (See Peter Sagan at the Olympics where he DNFd.)