Short answers, pulled from the story.
Three overlapping forces drove the shift: emerging modern ideals of selfhood, the declining fashionability of elaborate Rococo styles, and the rationalistic ideals of Enlightenment philosophers. The consumer revolution of 1750 had also already made fashionable dress accessible to all classes, so clothing became a means of personal expression rather than a marker of fixed rank.
The chemise à la Reine, also called the gaulle, was a loose garment of thin white muslin layers draped around the body and belted at the waist. When the painter Elisabeth Vigée-Lebrun depicted Marie Antoinette wearing it in a portrait, critics objected that it resembled women's underwear or boudoir loungewear, making it indecent and unbecoming for a queen whose clothing was supposed to express the dignity of the monarchy.
The Revolution turned dress into political declaration. Elaborate high fashion was associated with royalty and aristocracy, so radicals and Jacobins adopted plain, dark clothing and short unpowdered hair as a deliberate anti-fashion. The sans-culottes, whose very name referenced the rejection of aristocratic knee-breeches, made long trousers a symbol of egalitarianism, and French citizens were expected to wear the blue, white, and red of the French flag, often as a cockade.
John Quincy Adams, born in 1767, wore a short haircut and long trousers at his inauguration in 1825, making him the first president to break from the older fashion. The five presidents before him, from George Washington through James Monroe, had dressed according to the older style. Monroe earned the nickname "The Last Cocked Hat" for holding to it.
Panniers were hooped structures worn under skirts to push them out wide to the sides, a defining feature of formal court dress. They disappeared from everyday wear by 1780, surviving only at the most formal court functions. False rumps and hip-pads were worn briefly as the silhouette transitioned toward simpler, less structured shapes.
The 1770s saw wigs and natural hair built to extraordinary heights, sometimes incorporating symbolic decorative objects. One famous engraving showed a woman with a ship, masts and sails complete, built into her hair in a style called the "Coiffure à l'Indépendance ou le Triomphe de la liberté," marking a naval victory in the American War of Independence. These elaborate constructions were widely satirized in caricatures of the period and gave way, by the 1780s, to elaborate hats rather than elaborate hair.