With physical retailers, I have to hope I can convince the retailer to stock it, often have to pay up front for shelf space, and then have to continue to pay co-op fees for advertising services of dubious value.
Moreover, with Amazon, I know exactly how many units have been sold at any time and have the ability, for a fee, to get feedback from the buyers. With retail, you only get a rough estimate of sales, usually lagging by a quarter, and have no easy way to survey buyers.
I'm not saying that Amazon is acting like a great corporate citizen, but having worked with Amazon and most of the major national retailers, I'd take Amazon in a heartbeat.
It might be that personalization is in effect here (or some sort criteria), but I sometimes would like to better filter marketplace offers I don't even trust.
Amazon controls a large but minority of pretty much any market their in, until you start making smaller and smaller market definitions to the point of being ludicrous. As such, not being a monopoly, they're perfectly fine to do all this they want.