Why didn't plant indigo stay the main blue for mass-market jeans?

Answer: Synthetic indigo scaled

Synthetic indigo scaled ✓ — Right. Natural indigo had romance, but mass denim needed reliable, cheap, consistent supply. Once indigo became factory-made, mills could buy standardized dye instead of depending on variable crops and vats. Denim references still describe synthetic indigo as cheaper and more consistent for modern production, so the fashion color became an industrial supply-chain story.

Plant dyes were too bright — Not quite. The issue was not that plant indigo looked too bright for jeans; natural dyes often varied too much in purity, cost, and consistency. In mass production, repeatable shade matters as much as beauty. A mill making thousands of bolts needs the same blue again and again, not a charmingly unpredictable batch.

Plant dye weakened cotton — Not quite. Indigo dyeing can be demanding, but the historical switch was not mainly because plant indigo ruined cotton. Cotton had long been dyed with natural indigo before synthetic supply took over. The bigger industrial advantage was that synthetic indigo made the same dye available at scale, with fewer agricultural and fermentation variables.