Advertising/marketing.

People often perceive a mattress as "very important," as sleep is important, and are willing to spend a bit extra if they think the quality will be there and they'll sleep better.

In reality I believe most of these "mattress in a box" companies are selling the same 2-3 mattress manufactured in the same factory.

Personally, I buy mattress in the $300-$500 range. When I was in college I just slept on a brick of foam because that was what I could afford.