I'll take a stab at this, a brief one though. I was a Classics major in college. One thing I was interested in was ancient clothing.

From a western (Euro-centric) view, trousers were worn by people the Greeks considered barbarians, like the Bactrians and Armenians. They were also worn by their arch enemies, the Persians. The Romans carried on this tradition. They considered trousers effete and barbarous (Lever, James. Costume and Fashion: A Concise History. Thames and Hudson, 1995, 2010). Eventually, as Rome took over more of the world, trousers were more practical for men when riding horses, working, and for warmth. Byzantine (Eastern Roman Empire) court dress still consisted of robes, but trousers being more often worn by men, since they were more practical. When you look at what men were wearing it was often hose with a short or long robe over it. Sometimes two pairs of hose were worn: a tight under pair and a looser over pair. Eventually, this became "long johns" and trousers.

As for women, in many ways before modern tampons and sanitary napkins, skirts made much more sense. As a modern woman, I can attest to this. If one menstruates for five days a month, it is cleaner and easier to wear a skirt. For woman, urination is also more practical with a skirt. Men can "just whip it out," women have to squat. Also, if a woman is pregnant, a skirt or dress that can be easily adjusted makes more sense. Eventually, what is common becomes traditional. From a Biblical point of view (Deut 22:5) “The woman shall not put on [the weapons/armor of a warrior], neither shall a [warrior] put on a woman's garment: for all that do so are abomination unto the LORD thy God.” This came to mean that a woman should not wear the clothing of a man, i.e., trousers. Hence, we get a traditional view that trousers are for men, skirts for women, and kilts for the Scots.

edited for clarity.

Alrighty guys, putting on my mod hat here for a reminder of what posts here entail :)

First, I want to preface this by noting that yes, we know that this is on the front page of the sub and yes, we also know that there are no quality answers in this thread. That's okay. If you cannot contribute an answer that conforms to the standards that we uphold in this subreddit, then please refrain from posting - all it does is clutter the thread, necessitating us to prune it. Using popular history is not a source. A few lines is not a good post. Something that you heard once at a museum is also not an academic source. If you'd like further clarification on what we'll remove on sight, please feel free to check out the rules in the sidebar! :)

We do not lower our standards for popular threads. Please keep that in mind.

Not to be a self-aggrandizing douchenozzle, but let me link to my own post in another thread, showing that men wore skirts for about 3000 years, commonly, in Egypt.

This was also the case in Ancient Assyria, and, from the relatively scant evidence we have, in ancient Babylonia and Sumeria, as well.

Classical Greeks and Romans wore tunics, which were not skirts, but which did not have divided leg-like parts, and Roman soldiers wore a "skirt" of leather feathers (or pteruges) around their leather underpants.

Skirts were very common as a male garment, in other words, for thousands of years.

There are a variety of theories as to why pants took over. One possibility is that the advent of horseback riding, especially in long-distance travel and combat, necessitated a garment that would minimize chafing of the legs both against the horse and against any grasses or leaves, etc. that one would ride around against.

A question to add on:

I've heard a few times in the past that women's skirts, as well as high heels and longer hair, are all basically motivated by reducing the mobility of women (just as OP points out a skirt would do) and women were denigrated for adopting the more practical "male" modes of flat shoes, cropped hair, and pants.

Any truth to that? I know that Catherine de Medici was the one to introduce high heels, and I think it's also true that men have worn their hair long at various points in history, but it doesn't necessarily negate the point.

Anne Hollander's book Sex and Suits argues that the first truly tailored/fitted garments in Western history was the cloth padding worn under armor by knights and other fighting men. At this time, both men and women basically wore bags that weren't shaped to the body. The padding suits gradually influenced military uniform, which in turn influenced government fashion, which in turn influenced menswear in general. The older style did hang around on men, particularly in the uniforms of institutions that wish to project stability and authority, i.e. the gowns or robes worn by priests and judges.

As women were not part of the new institutions of the modern European period, they did not adopt the body-fitted fashions, and retained the older "bag" paradigm in the form of dresses and skirts. You often did see women wearing fitted garments on their upper bodies, though.

In Western fashion history, it's men who are the innovators, and women pick up bits and pieces of it as men abandon them. Women didn't really start wearing pants or trousers in public until the 1930s or so, revealing at least the shape of their lower bodies for the first time.

Practicality really isn't high on the list of reasons why people wear the things they do.

Additional question to this subject, is the shaving of legs by women related to their wearing of dresses/skirts or unrelated. If it's unrelated I'll just open a new question about it.