I’m just guessing, but maybe because shops want to earn money. It’s a fact of life that thin people spend more on clothes, not because they are thin and have more money, or because they have greater choice and just have to have everything they desire, but because they instinctively care more about their appearance (The ones who buy a lot of clothes do anyways.)
I’m a size 18 (maybe a 20 in some stores) and I miss some of the shops that I used to visit that have fallen by the wayside.
But bottom line is shops are in the business of making money, not carrying dead stock just on the off chance a larger customer comes in, and they are certainly not in the business of commissioning special designs for their larger customers. It a loss making exercise, and they are not going to write losses just to be PC.
When I used to go on holiday (many years ago) I used to buy a tonne of clothes in Florida. Their population is larger and they have the customer base to make clothes for larger girls. One year I bought so many clothes we had to buy another suitcase to get them home.
Its not discrimination. It’s perfectly logical. I shop for clothes maybe twice a year, and usually online. I’m also short, so it’s an added challenge.