There are all sorts of outlets for reasonably priced, what you see is what you get mattresses: Sam's Club. Costco. Ikea. You can get a decent name brand mainstream mattress for ~$500.

The problem with mattresses is that it's a product where you need a salesman. There's a bunch of different products that look similar, but have significant differences. You want to match the customer with what they want (or tell them what they want) so that they don't return the thing.

Plus, there are some inherent logistical differences between a pair of eyeglasses that can be dropped in an envelope and shipped to anywhere in a day or two. Distribution and warehousing is expensive, the product needs to be delivered quickly (and picked up if the customer is displeased).

There is a market for discount mattress sales outlets on the internet -- but just as online furniture and appliance outlets haven't "disrupted" the market, mattresses online are unlikely to either.