Answer: Actually, this is a very old tradition stemming from verses in the old testament banning cross dressing.
This includes men wearing dresses and women wearing pants. In fact, this is why women never wore pants until relatively recently... he was seen as a crime against God. Most orthodox/semi-orthodox groups (including Greek Orthodox, Amish, Mormon, Quakers, Catholics, and Jews) still follow this ban relatively strictly. Only in contemporary culture have women been allowed to "dress as men" and wear pants. Though, hypocritically enough, men are still generally prohibited from dressing as women.