I think one of the things that makes The Rose of Versailles so special is the context it was written in. Riyoko Ikeda, along with other members of the Year 24 Group, were changing the face of shōjo manga:
While shōjo manga of the 1950s and 1960s largely consisted of simple stories marketed towards elementary school-aged girls, works by members of the group significantly developed shōjo manga by expanding it to incorporate new genres, themes, and subject material. Narratives and art styles in shōjo manga became more complex, and works came to examine topics such as psychology, gender, politics, and sexuality
In addition to being a part of this shift and writing a more complex story for women and girls dealing with politics and exploring (queer!) gender and sexuality, Riyoko Ikeda was inspired to write about the French Revolution based on her own experience with leftist politics in Japan:
Riyoko Ikeda came of age in the 1960s, a decade that saw the rise of the New Left in Japan. This political movement, inspired in part by the ideals of the French Revolution, galvanized Japanese youth and led to the formation of student protest movements. Upon entering university in 1966, Ikeda became a part of this movement after joining the Democratic Youth League of Japan, the youth branch of the Japanese Communist Party.
When writing and publishing The Rose of Versailles, her editors were not that into the entire thing, so she relied on feedback from readers to drive the direction of the manga:
As the New Left declined in the early 1970s, Ikeda decided to create a manga focused on themes of revolution and populist uprising. After researching the French Revolution for two years, Ikeda proposed a manga series that would be a biography of Marie Antoinette to her editors at the Japanese publishing company Shueisha. Though Ikeda's editors were reticent about the concept, the first chapter of The Rose of Versailles was published on May 21, 1972, in the weekly magazine Margaret. As a result of this lack of support from her editors, Ikeda frequently relied on feedback from fans to determine the direction of the story
I just get fired up thinking about Riyoko Ikeda and her contemporaries coming in and asserting that girls don't just want to read stories about family drama and romantic comedy, they want to read stories about politics and psychology that explore gender and sexuality with nuance and complexity. This on top of the fact that before this point "authors of shōjo manga were typically men who began their careers in the genre before migrating to shōnen manga, or manga for boys" makes this era feel like a powerful redefinition of shōjo.