I seem to remember something about how the Black Death contributed to the rise of hose, then later "hosen" or pants. Went something like this; bare legs were seen to be an obvious in road for the "daemons" that were killing people. By wearing hose (high stockings) less of the leg (and less of the "other portions") were visible, and thus less attractive to the foul spirits. by covering more and more of the body the "spirits" would have less to be attracted to; hence the idea of modesty as a prerequisite to "holiness".

Of course the farther north you went, the less likelihood of running into the rats and fleas that carried the Plague, so the far Highlands of Scotland did not see as great of infestation as more southern climes. This held true till the advent of the woolen mills and more crowded conditions made the "plague" a more common pestilence. Remember plague was one of the main reasons Cromwell fell into disrepute, deemed as a judgment of God on his "revolution". Of course the same held for the Restoration period; until public sanitation and routine hygiene decreased the flea to human transmission rate; but by then the custom of pants had been established.

I believe the Equestrian argument has more to do with mounted armour than with ease of riding. Many cultures well versed in horsemanship did not adopt the "pantaloon", till well after the advent of the mounted knight, especially a Knight in full Plate. American natives used at the most, a form of chaps, and certainly not full pants, until late in the 19th century. As the influx of European culture marched across the continent, the natives adapted as well. Here in New Mexico there are many early 20th century pictures of tribal elders in the traditional robes, and garments that look like unpleated kilts (skirts).