I think pants were introduced by eastern peoples who used them for riding horses. Their use would have become common in roman-barbarian kingdoms, i think. Anyway, they are obviously better for riding and for rough terrains, where larger tunics would become trapped by plants, spikes, and other similar.

All of this is IMHO, i've no historical evidence of that.

@Aemilius- they wear their standard clothes, i think. Culture is hard to change, even if you are freezing your jewels XD