The kilt was solely Highland Scots attire, the lowland Scots were, until very recently, non kilt wearers. In fact they were exceedingly vociforous with their scorn of the Highlanders and their kilts.
That's a fair and valid point. And it leads to a question, while we're on the subject.

When the kilt was officially made the national dress of Scotland, did it pertain to all of Scotland, or just the Highlands? And if it was for all of Scotland, was it meant to encourage Lowlanders to wear it as well, even though it wasn't their custom?