Just based on my anecdotal reviews of eBay, etc., it seems like there are lots of vintage kilt jackets that lack a waistcoat, fewer kilt jackets that have an accompanying waistcoat, and even fewer unaccompanied kilt waistcoats. Does anyone know the reason for that?

Given the local climate, I'm more likely to want to wear a vest/waistcoat than a jacket (for most of the year).