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.