I've been hiking only in long skirts since I was a teenager.
The biggest reason is coolness combined with protection against Lyme Disease -- a whole lot of fabric flapping around my ankles is such fantastic insect protection that I don't have to bother with anything else on my legs and feet. Plus all that moving fabric helps keep me cool, and since my very pale legs are covered, I don't have to keep stopping to renew sunblock.
But then I continued to hike in long skirts in winter, which wound up being a lot more practical than it sounds -- I'd layer whatever I liked underneath and throw the big skirt on top, though for snow I'd switch to a shorter outer layer. The skirt (or skirts in layers) would act as a windbreak layer and hold a tent of warmer air inside, and then I'd have whatever thermal layers I wanted under that. Conventional instructions on winter layering always make me wind up with so much fabric and padding wadded between my legs that I wonder how anyone walks.