because she was tired of being “stuffed & trussed” in Court costume. Marie Antoinette escaped to her “Petit Hameau” (little farm) at Versailles, were she wore her white muslin undergarment called “chemise a la reine” (underwear of the queen).
(photos: “Petit Hameau” and Marie Antoinette in about 1790 wearing a “dressed up” version of her undergarments.)