Why? Because cotton jeans seem to attract the cold and the wind whips right through them. But flannel-lined? Ahhh, the flannel feels warm against the legs, especially when you have skinny chicken legs like mine. And the flannel helps combat the cold north wind. Furthermore, you can wear them indoors as well as out and dial down the thermostat a bit. But even if you don’t lower the inside temps you won’t overheat and drop from heatstroke. Lastly, no one knows you are wearing them. The stiffness of the cotton fabric helps the jeans keep their shape. (I have worn flannel-lined docker pants and they don’t look nearly as good, although I am at that age where I’d rather be warm than look stylish.)
I’ve gotten mine from Penney’s and Eddie Bauer (when on sale) but the Duluth Trading Company has them at a good price. So, if you are looking for a gift to give or need to put something on your holiday gift list, think flannel and think warm.