It's not so much that I'm *required* to wear skirts as it is that the rules for pants are a lot stricter. Pretty much any knee-or-below skirt is acceptable, but pants have to be certain colors/materials in order to be "professional." (The owners apparently like... have the worst hate for jeans or something, so they went all-out on trying to make sure no one would dare attempt to sneak a pair of something-even-kind-of-like-jeans into an office.)
They can also be strict on clothing appearing "untidy", like wrinkled or creased in the wrong places, and I don't have an iron! With skirts, the material often doesn't wrinkle/crease, or if it does, it blends into the fall of the fabric, haha.
I did have a couple pair of slacks that I used to wear, but they don't fit terribly well anymore. :/