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.
In addition, due to risk of bed bugs/contaminants and the fact that people are very particular about the comfort level of a mattress, the re-sell market for mattress is really unlike other pieces of furniture. Whenever I've moved, I've really only been able to sell mattresses to people who trust me because I can vouch for living in bed bug free zone. Otherwise, I've just had to trash it because as you mentioned, it's way too cumbersome to transport.
That's kind of why certain furniture brands catered to urban markets (smaller dwelling) tend to make some of furniture able to be dis-assembled: much easier for moving around.