I have two hypotheses on the matter:
1) This "rule" is in place because none of the men making the rules ever has to wear things like dresses, skirts, and pantyhose and does not know how it feels to wear stuff like that in various situations where it is very uncomfortable. For example, the wife tells me that summer time is definitely not a good time to wear hose. And wintertime is much more comfortable with pants.
And the argument about men having to wear suit jackets in the summer is not valid because a lot of congregations will make an exception on hot days that the men don't have to wear them. There are no exceptions to the dress/skirt rule for women, however.
I have only seen one exception to a sister wearing a dress/skirt to the TMS/School meeting. She had to work close to the meeting time and had to wear a uniform, so there was an exception made for her special case, except for when she had a part on the school. Then she would have to dress appropriately. She wasn't considered very good association by many for this and other reasons, however. That is sad.
2) The witnesses work for a publishing company. And most publishing companies have dress codes for their sales staff and their office staff, don't they? Gotta keep up that corporate image, ya know.