When none of the wheels of a bicycle are on the ground, the center of mass is going to accelerate downward at approximately 9.8m/s/s and there's nothing the rider can do to significantly alter that. If both wheels touch the ground while the center of mass is at the highest possible point where that can occur, there will be a severe limit as to how much further down the center of mass can move after that point without damaging something. This means that, if landing on flat ground, the wheels must transfer enough force to the ground to reduce the velocity of the center of mass to zero within that distance, or else the center of mass will travel further than would be possible without damage, and as a consequence something will get damaged.
While the rear wheel is on the ground but the front is not, any force with which the rear wheel pushes on the ground will help to reduce the downward velocity of the center of mass. Although the rear wheel alone cannot supply as much upward force as would both wheels together, the center of mass can be much higher when just the rear wheel touches than it could be with both wheels touching. This effectively increases the distance over which the velocity of the center of mass can be reduced to zero, and thus reduces the peak amount of force required to accomplish that.
If a bicycle is landing on a downward-sloping ramp, the downward slope of the ramp will reduce (sometimes significantly) the amount of velocity the bicycle will need to shed after making contact with the terrain, to the point that one may be able to make a comfortable two-wheeled landing after falling a distance which would make a two-wheeled landing on a flat surface injurious to the rider, bicycle, or both.