Industrialization. For eons, only wealthy people could afford to hire staff to do the work of removing the bran, finely grinding the wheat and making soft white bread. Naturally, people who couldn't afford it wanted it as it represented a luxurious lifestyle rather than the life of hard work and poverty that most people experienced. Once the process was mechanized, it became available to more people for an affordable price.If its a joke, how do explain that after eating dark bread for eons, some people started eating white bread, which eventuality pretty much took over the non- peasant world?
White bread flour strips out most of the nutrition of wheat.
The reason is underwear. French peasants wore no underwear or black underwear. This needed infrequent washing (think about it). Capitalists and the aristocracy started wearing white underwear because it required more frequent washing which showed they were so rich they could afford daily laundry. Peasants also ate dark, fully nutritious, bread, so the capitalists starting eating white bread to show they were economically superior to dark bread eaters-- it cost money to destroy the nutritional value of wheat (dark bread).