I concur that the Caldwells never acquired a tartan of their own.

Why is it that the lowlander Caldwells never had a tartan, when their lowland neighbors, the More, Wallace, Stewart, Cochrane, Sempill, Montgomerie, and Cunninghame families, all did?

I do not know. Maybe it is the fact that these lowland families listed above all had a "Sir," registered coat of arms, family crests, and the like, and the Scottish Caldwells did not.

Tom Caldwell points out that wearing Highlander tartans became fashionable among lowlanders rather late, during the 18th and 19th century.

Unlike the Sirs of More, Wallace, Stewart, Cochrane, Sempill, Montgomerie, and Cunninghame families, the Caldwells of 18th and 19th century Scotland had risen in social status only to mere Esquires, the lowest tier of gentry. For example, William Caldwell of Straiton was a mere Esquire.

It seems that the tartan was reserved for the kilt, a ceremonial apparel, rather than adopted for daily clothing. Many of the Caldwells of 18th and 19th century Scotland may have been too frugal to buy a kilt for occasional ceremonial use. This was an age when vast numbers of Caldwells were migrating to distant lands.

It is ironic that the Caldwells were one of the more common families of Kilmarnoch in Ayrshire, whose mills wove many of the highlander tartans.