“Because someone did research and found that indigo dye is easily removed from a material that it has been set into”
The indigo dye molecule binds better to some fabrics to others. It binds very well to wool — I’ve seen techelit strings that are over 20 years old and look brand new. It does not bind as well to cotton, hence fading of blue denim over time. It does not bind well to most synthetic fibers.