The Sewing Machine was invented in the mid 18th century and was more or less perfected by the middle of the 19th. As has been discussed elsewhere, trousers can be made easily with a sewing machine, but a Kilt requires more hand work, presumably even if the same fabric is used... I agree, there are many factors at work, certainly including a taste for "modern" ways, a desire to appear less rustic, etc. I would imagine many kilts were worn well past the lives of their original owners, but they do wear out eventually- and a worn out kilt could be reworked into a pair of trousers, couldn't it? And some careful thinker might be able to address the causes and effects of underwear in this evolutionary process... Finely knitted drawers and union suits? Comfortable cotton shorts to protect the wearer AND his woolen trousers?