But how do they control it? Is it just that they make a good product at a market-clearing price, or do they abuse the system somehow? In my mid-sized American city there are not just one but two local mattress makers that I could buy from if I felt like I needed to, and I did once but I don't feel like the choice was either denied to or forced upon me.

A lot of it, if I'm correctly remembering an article I read several years ago, is that several of the most popular brands are actually the same company, and that company also quietly owns several of the most popular chain stores. So you go into a store you think is in competition with the store across the street, and look at mattresses from manufacturers you think are competing against each other, and it's all one company owning both stores and both mattress brands.

It's not as universal as Luxottica, as I said, but it's pretty widespread.

And if I recall rightly, the manufacturers made different products for each vendor, making it very difficult to say, "I like mattress X from company Y, let's see who has the best price on it."

This led to a lot of people having favorite hacks. E.g., a friend bought his mattress from a hotel company. His theory was he was probably paying a little more, but the incentives were better, so he trusted the hotelier more.

There a lot of "if I recall correctly" answers but no one is listing any company names.

Who is this Mattress Powerhouse? From whence they came?

I'll tell you what. You tell me the things you've Googled for, and how the results fall short in terms of finding things that explain the mattress market structure. And then I'll correct whatever issue leads you to not finding the stuff that's out there. Deal?