Robustness is another aspect that differentiates road and mountain bike saddle requirements. It is a reality that we do occasionally crash on singletrack and you are more likely to have a bike cartwheel away from you off-road than on-road.
That means that a tougher and more wear-resistant material composition is desirable for your mountain bike. Singletrack riders are also more likely to ride in extreme rain and mud, which requires a saddle that is comprehensively weather sealed.
The body and rails of the saddle also need to be structurally strong to withstand seated pedaling over rough terrain as well as impacts in a crash. If a rider loses control or their feet come off the pedals, the saddle will often be the first point of contact and need to be able to withstand a rider's entire body weight landing on the saddle.