Why are aero bars usually disallowed in Gran Fondos (cyclosportive events)?
I'm just curious. I would think it would be a useful piece of equipment when riding 100 miles.
I now realise "gran fondo" is American terminology for "cyclosportive". That sort of event falls loosely between audax and racing. The randonneuring community prize responsible riding, and tend to frown upon behaviour or equipment that could be deemed "antisocial". For example, most audax events require riders to fit mudguards, for the benefit of others.
In "gran fondos", although riders may only be competing against their own personal goals, they may find themselves sharing the same stretch of road with others, and some participants may wish to treat the event as a private race.
Steering control is severely compromised when using aero bars. They're not considered safe when riders may be racing in close proximity - a slight wobble could cause a major takedown. They're not great for cornering either. Hence the reason they're illegal in most events, except triathlon and time trials, where riders are mostly spread apart and riding fairly straight courses.
@headeronly has the best answer, but this is a great supplementary quote from The Rules: The Way of the Cycling Disciple by The Velominati. It's a tongue-in-cheek book but illuminates the problems nicely.
For a while, aerobars were allowed in one-day races and road-race stages of Grand Tours. But this presented a new problem; when a rider’s grip on the bars is narrowed into an aero position, their hands are far away from the brakes, the steering of the bike is compromised and controlling the machine in a situation where there are up to 200 people riding butt to nose to shoulder, it becomes much more of an issue than if you are barreling into a corner at speed, all alone and can pick whatever line you please. While the type of aerobars were more compact than those typically used in a time trial (i.e., Spinaci bars), there was an increase in sketchiness within the peloton that couldn’t be ignored. Mostly on the grounds of safety, they were banned from mass-start events.
Well, the simple answer is that aerobars aren't very helpful unless you are riding at reasonable speed (say, >18MPH or so), but most people who ride at that speed would prefer to ride in a paceline, which gives more benefit than aerobars.
Aerobars are unsafe in pacelines because riders cannot maneuver as well as being on the drop bars.