I think the explanation is simple. Many other stores are now open that celebrate the larger sizes. Often by the same companies that give us the skinny stores. These stores are just for us. I am cynical enough to believe that this decision, to separate the biggies from the thinnies, is not based on what we want or need. They are separating us from the other stores because they don't want us in there.
What image do you get of a store that is frequented by today's average women?? Size 12, 14, 16, and more? Lower-class, right? More than that. Out-of-control slobs. Lazy people. A whole whale of assumptions comes with the territory of large-sized women. Women with no self-control. Women who will never get anywhere. Women who are morally deficient.
Most of these women would not be as large as they are if they were not dedicated to dieting, to exerting self-control until the dam bursts. By restricting calories they set off a series of changes in their bodies that make it near-impossible to lose weight and that cause them to gain more each time they fall off the sacrificial wagon. If they had never dieted in the first place they would likely not be rail-thin, but most of them would be thinner than they are now. The dieting itself has created them. This is but one of the facts of so-called "obesity" that is generally not acknowledged by the thriving community dedicated to weight-loss programs.
The real horror is that the basis for all of the billions spent on "obesity research" is erroneous. Being fat in itself is not unhealthy, does not lead to shorter lives. This leaves only the cultural phenomenon. And what a phenom it is. It's what keeps me out of those stores.