I don’t know either, but long denim skirts is the “uniform” some places.
Here, for example, and I have seen some incredibly ugly ones.
I don't understand the appeal of denim at all, except for hard manual labor situations. My husband wore jeans to work today (in IT at an investment company). It was a reward to his department for some kind of accomplishment: they were allowed to wear jeans on a Thursday.
Jeans are less comfortable than pants of other materials. Denim skirts are less attractive than skirts of other materials. Sewing with denim is horrible, and when you get the garments made, they take forever to dry!