If flowy, high-waisted gowns were the fashion by 1813, why then does the Queen still dress like it's 30 years in the past? The brilliant answer to that question comes from history itself, and the real-life Queen Charlotte upon whom the character is based. Namely, that she was considered an unfashionable, stuck-in-the-past grump even in her own time.
https://mashable.com/article/bridgerton-queen-charlotte-fashion