The looms play a big role in explaining why selvedge denim is more expensive. Selvedge fabric is made on older, less efficient machines called shuttle looms. These are more costly, time-consuming and arduous to use, but they produce better quality results (and, of course, those lovely selvedge edges).
On the contrary, modern, large-scale denim looms can mass-produce in significantly less time. However, they can’t create a selvedge edge, which in turn also makes the denim less durable.