I'd have thought that the major reason the kilt isn't commonly worn is that it's not fashionable. Regardless of where people come from, the larger part of people today wear clothes and follow styles that are more or less dictated to them by the fashion/entertainment industry. Culturally people do follow styles, you still see those in baseball caps, plaid shirts and jeans and maybe sleeveless jackets. You see people who wouldn't look out of place in a biker gang, you get those who dress in sports attire - which would suggest that people are taking cultural cues from tv and identifying with what they see.
I think it should be considered remarkable, both given the near absence of kilts in common entertainment culture and the price of them, that so many people today still wear them who aren't in pipe bands or do highland dancing.
Maybe a few more programmes like Outlander will serve to promote its use but I do wonder if it wont just reinforce the stereotype of it as an exotic garment for special occasions.

Another thing about the kilt, and I know some will not agree with this, is that by its nature of being heavy and longish it doesn't readily fit in with modern life, the kilt being more akin to suit trousers. Maybe it's different elsewhere but here in New Zealand, females seldom wear skirts and dresses these days for everyday wear. They do so for going out at night, weddings etc, but otherwise wear casual shorts or trousers. It's very similar to the kilt situation and females are associated with such clothes far stronger than males are to the kilt.
I'm not meaning to suggest that kilts and skirts are the same, but the use of them has seen them restricted to special events. Now obviously many women do wear skirts and dresses regularly, far more so than men wearing kilts, but something about modern life has made the wearing of functional bland clothing for everyday wear more appealing than traditional skirts and dresses.
If that can happen for women, then it's hardly surprising that men wearing kilts, which were never widely worn outside the military, is not as common as some think it should be.

All things considered, I'll go back to the observation I made earlier and say it's remarkable that so many men do wear the kilt. It shows the level of interest in it despite the lack of support for it by modern "purveyors" of culture.