Mountain bike saddles differ from road bike perches due to the environmental and frame differences that apply to these two cycling disciplines.
Mountain bike frames are generally longer than road bikes, with much slacker head angles and a contemporary trend towards very steep seat angles. You’ll be in a more upright seated position when climbing your mountain bike, which alters weight distribution over the saddle’s support surface.
It is also worth remembering that mountain bike drivetrains feature extreme climbing gears not found on a road bike. On gradients where you will be out of the saddle, powering in a standing position on your road bike, you’d be seated on a mountain bike, running a much higher cadence. This means that any possible discomfort will manifest much sooner when climbing your mountain bike.