Here's my explaination for the lack of pants. Witches all have tails and thus they don't want the trouble of dealing with pants when they sprout them. Failing that one's legs being bare or covered in a very thin layer of clothing(those we've seen wearing pantihose/tights) is conducive to the use of magic. Most women can use magic to varying degrees and so most of the women we see will be pantsless. The older women wears pants because they've probably achieved high control or the requirement for bare legs is lifted by some change in the nature of their magic as a result of their advancing years. All the Strike Witches are Witches so all have bare legs.In addition women who have stupidly low levels of magic or none at all may go pantsless in order to create the illussion they've got as much magic as everybody else.