Why do conventional flours chemically bleach their flour?

Before chemical bleaching was discovered mills used to let their flour sit for approximately 12 weeks to develop the gluten, through natural oxidation in from the air.  Since the chemical bleaching process was discovered mills  were able to mill and ship in a very short period of time without having to store the flour. This processes also removed all the great vitamins and minerals naturally found in the flour. This created the need to fortify the flour because it is such a staple in our diets.