It's a matter of cost and the inventory limit a retailer can handle.
1) From a cost perspective, the biggest investment with footwear is in the tooling. That's the midsole and outsole of the shoe. Metal molds need to be created for each half size and they typically cost $5-10,000 USD for each midsole+outsole mold. That's why manufacturers will use a bell curve and limit the size run of the model. For M's, it will typically be from US size 6-14. So that's already $170k USD for one set of molds. In regards to width, the molds can usually accommodate 2 widths. So if a mold is for a D width, you can use different lasts to get to a C width and E width. It's often tough to tell the difference between one width difference, so if you want to offer a 2E width and actually have the consumer feel the difference, then that requires a new molds, and now they have to double the investment with another $170k, which needs to be amortized over the additional volume of width shoes...and that number is never doubled. Tough to pencil out from a cost POV.
2) In order to get these width options to a consumer, the retailer has to be willing to carry this extra inventory and SKU. Knowing that the you might sell 1 2E or B width shoe for every 10, brick and mortar doors would rather carry a more efficient SKU (another model or color way). Online stores have less of a space constraint, but it's still about efficiencies.
Because of both conditions, it's chicken or egg, and both supply and demand are low. Niche companies like NB can hang their hats on widths, but its still a considerable investment and niche because the bigger brands feel ok leaving that business on the table.
