There is a wonderful book titled ~An Elegant Madness, High Society in Regency England~ by Venetia Murray. In one of her chapters , she discuses this very topic.

“ The year of the Regency period saw a complete revolution in dress for both men and women. As James Laver, the great authority on English fashion, wrote, ’ For a whole century the accepted forms of male and female costume had remained fundamentally the same. Once the tradition was broken, anything might have happened.’ What actually happened was the direct result of the French Revolution.
In their hatred of the embroidered garments of the nobility, the men and women of 1789 turned, on one hand, to the English country fashions ,and , on the other, to what they imagined was the clothing of the ancient Greeks. The result was the top-boots for the men and a single flimsy, chemise-like garb for women became the accepted wear……..people only began to adopt the costume of the ancient world when the ideals of Republic virtue had been abandoned for the frank pursuit of pleasure. The frivolity which the Republic thought they had extinguished for ever with the blood of aristocrats burst forth with renewed intensity once the threat of the guillotine was forth removed…women found themselves suddenly emancipated, and their first reaction was to cut their hair short and to take off most of their clothes.”

As for Brummell, he was at the right place at the right time, so to speak. Because the rules of fashion had begun to sway from the Fop, to a less elaborate style, his subdued elegance and eye for fine tailoring aquired him a reputation. Although his style was more simplistic than those before him, it is reported that it took up several hours to make his toilette. Who says simple is always easy.