Probably several reasons. The poor quality cushioning (couch-grade polyfoam) loses about 40% of its resilience in the first 18 months. If it was an extra-thick, extra-padded mattress, research shows that the more plush the padding, the faster it breaks down. Also, many U.S. manufacturers are not making higher innerspring coils for these extra-thick mattresses. They're just stretching out the standard coil. So it loses its compression and shock absorption properties relatively quickly.