Here's an asumption:

When you get into the overweight category, sizing becomes a lot more difficult, because then the ratios that are relatively standard for non overweight bodies (like waistline to tallness) completely break down.

So now you don't need just one parameter, but at least two: waist and tallness. And this causes the number of different sizes to explode. So instead of (S, M, L, XL, XXL, XXXL, XXXXL), you'll need (S, M, L, XL, XXL, XXXL) waist x (S, M, L, XL, XXL, XXXL) length. And it becomes unmanageable for brands to cover all the different sizes without having a large amount of waste.

Men trousers and shirts do this, because generally men fashion don't change that fast, so the brands can carry many models for a longer time period.