I always enjoy a simple: "Because cops frown on public nudity."

When facing crude, idiot college guys it's usually: "Because they don't make three legged pants."

When someone is implying it's feminine: "I saw it in my mom's closet and it just looked so comfortable"

When asked if there's an event: "Kiltfest January 1st - December 31st"

Take it however you want: "You ever see that movie Highlander? It's just like that."

For the moral highground: "It's about freedom... Hey, get your mind out of the gutter!"

Also fun: "It's to protest [insert bad thing here]." (for extra credit/screwing with friends, expand into a full on scene causing, loud, embarrassing, fake rant. Not particularly kilt oriented, but fun)