I don't know about kilts becoming mainstream but I have noted something about men's fashions. For decades it seemed that men were more tightly bound by convention in terms of what they wore and how they wore. Hairstyles, clothing, hats, etc were all within fairly rigid parameters. Nowdays there is more acceptance for men to have a broader range of fashion options. For examplee (at least in California) it is perfectly acceptable that Gentleman #1 has a businessman's short haircut, Gentleman #2 wears his wavy locks down to his shoulders, and Gentleman #3 has shaved his head bald. The hair style doesn't say anything about the wearer other than that is how he likes his hair cut. The same with clothing. So now Kilts can be perceived as a fashion option because men now have more fashion freedoms.
Still a loooooong way to go before they go mainstream, but it's a start.

Cheers