I don't know if this has any impact, but thinking about it, a mattress is a possession that can't be easily transported in a car, or even, by someone who doesn't have a car. I imagine that as people become more mobile and choose to have fewer possessions, it might be easier to simply abandon their mattress and have a new one shipped to their new location.
Pretty much as soon as you buy a bed, your car is too small. And if you're at the level of self sufficiency where you own a bed, you probably own other things like chairs, tables and chests of drawers.