In Blue Jeans: The Art of the Ordinary, Daniel Miller and Sophie Woodward look into the cultural values of blue jeans beyond the ordinary justifications for preference given by wearers - and they find that there's no single good answer.

It's true that originally, indigo was simpler to use for dyeing (it doesn't require the fabric/yarns to be pre-soaked in a mordant, a chemical that helps the fibers hold dye, though it does require more of an effort to make the pigment usable), but by the 20th century synthetic dyes were overwhelmingly being used in the garment industry and there was no reason to continue making them blue. Essentially - and delving a bit more into anthropology than history - it seems that because the original blue jeans were blue, the values associated with denim pants have been assigned to blue denim specifically. The way that natural indigo faded became a signifier of use and personality, and therefore something to be imitated with synthetics.