This is not just a matter of clan loyalty: kilts are more comfortable than trousers in the warmer weather.
But there are some social issues: Peter Turchin observes,
"To convince you of the primacy of social factors I urge you to check out this extremely funny site, Bravehearts in Kilts Against Trouser Tyranny:
"This site is hilarious not because the Bravehearts in Kilts are stupid, but precisely for the opposite reason. Once you have read their passionate defense of the kilt, you (at least if you are a male) will realize that it is us, pant-wearers, who are stupid. In warm climates or during summers in the temperate zone the kilt is much more comfortable to wear than jeans."
I have put off buying my summer kilt until I have figured out the right green-shaded clan tartan. Don't want to get on the wrong side of some exceedingly ferocious fighters!
Nobody in antiquity wore trousers except barbarians. Peter Turchin again:
"The basic garment worn by the Greeks was the chiton (basically, same as the Roman tunic). And wearing ‘sacks’ around their legs was something that only barbarians did. The Romans of the Classical Age felt the same way. Citizens were required to wear togas for any official functions, and at other times (e.g., for war) they wore tunics.Part II of Peter Turchin's illuminating article here.
"Why did the Italians switch from tunics to pants? The answer is the horse. Not only are the horses responsible for why we live in complex, large-scale societies (or, at least, how such large-scale societies first evolved), they are also the reason why males have to swelter in pants in summer, instead of wearing the cool kilt. As I will discuss in my next blog, there is an exceedingly close historical correlation between the adoption of cavalry and switching to wearing pants."