The bumpers are usually made of soft material, the idea behind them is to "protect" infants from injury by hitting their heads against the side of the crib, als

The bumpers are usually made of soft material, the idea behind them is to "protect" infants from injury by hitting their heads against the side of the crib, also because prior to regulation changes in the 70's crib bars were often spaced far enough apart that limbs could get caught between them. Soft material is the problem, as an infant may roll into the bumper and the material will shape itself around the face and mouth causing asphyxiation. Cardboard boxes are not malleable, obviously, and even if an infant rolls on their side right next to the wall of the box it is impossible for them to do so in a manner that would hinder their ability to breath (the nose will get in the way, or they will retain the ability to breathe through the sides of the mouth even if they manage to completely put their mouth against it).