Indigo Wears Off

Buchanan's A Weaver's Garden is a useful book for making the connection between plants' properties and how things came to be, or why certain products have traditionally been made in a certain way.

For example, she notes

[indigo] doesn't react with the fibers, it just adheres to their surface, forming a mechanical but not a chemical bond....Indigo doesn't fade over time or change from dark to pale blue, but indigo-dyed fabrics (such as blue jeans) gradually fade as the dye molecules are rubbed off the fibers, like chalk marks are erased from a blackboard.

Rita Buchanan, A Weaver's Garden (Loveland, CO: Interweave, 1987) p. 117.

People came to expect jeans to lighten with wear and found softer, worn jeans more desirable.

Consequently, producers began to simulate mechanically worn indigo's look by treating jeans with rapid chemical and mechanical processes.

I'm sure you could go look up information on the environmental impact of modern processing for blue jeans and get all horrified, but I'll just leave you with the thought that the process (and impact) is driven by long-standing expectations people have of indigo, based on its particular natural properties. I think that's pretty neat.