Generally, adding fairings and other modifications to improve aerodynamics also increases the side-on area susceptible to crosswinds. For uprights the rider is

Generally, adding fairings and other modifications to improve aerodynamics also increases the side-on area susceptible to crosswinds. For uprights the rider is seated such that the centre or mass is relatively high compared to the faired/aerodynamic frame/wheels. If the bike has fairings or wheels covers that increase the side-on profile then a strong wind gust can push the bike out from under the rider. For this reason front wheel disks are banned from time trials and triathlons. There is too much risk that a crosswind gust can destablise the bike and rider. For recumbents the rider's mass sits much lower to the ground, more centrally within this faired frame/wheels, so they are inherently more stable to the effect of crosswinds. While I don't agree with all the UCI rulings, some of them regarding aerodynamic modifications are made with safety in mind. For example, triathlon style aerobars are banned from the peleton. This is primarily because the rider has less control over the steering and braking performance while in the aero position.