Leaving all historical issues / discussions aside:

When you cut anything on the bias, the material has some 'give' or 'stretch' to it. Try it yourself...

Take your kilt and follow a line and pull STRAIGHT ACROSS that line. Now turn it 45" to the diagonal and pull. You'll see that the material stratches on the diagonal, but not straight.

This is a neat property of woven cloth (not JUSt tartan, but all woven cloth).