Widows have always worked and carried on their husbands' businesses, and women left without support or assets have always worked because they need to eat and wear clothing. Gone with the Wind itself offers the example of Mrs. Merriweather selling pies she baked at home, Mrs. Meade teaching school, Mrs. Bonnell teaching music, and Mrs. Elsing working as a seamstress. (p. 887) Mrs. Merriweather eventually expands to a bakery downtown employing six people. (p. 1077) (Both page citations from 75th anniversary edition Scribner eBook, with preface by Pat Conroy.)

The more revolutionary parts of the work of Scarlett O'Hara were that she was earning money in an exceedingly undomestic sphere by taking an active role in management while married to Frank Kennedy, and then, even more shockingly, while married to an extremely wealthy man, Rhett Butler. Women with the right to hold their own money have always invested it in property, businesses, and so on, but taking an active management role was the anomaly.

The antebellum South would allow many upper-class women to maintain a protracted state of mourning, protected by male relatives. (going back to Gone with the Wind, p. 186 details the seclusion and lack of socializing for three years) With few male relatives of a useful age, women were either forced to find a way to make some income or were likely to die, and in fact, malnutritive disorders such as pellagra or chronic problems like tuberculosis would strike down those who were left with little protection.

The extreme social roles prescribed at the time before the war, and therefore the predominant guideline in the confusion of post-war society, were the true barrier, as Rhett Butler told Scarlett on p. 946, pointing out that "All you've done is to be different from other women and you've made a little success at it. As I've told you before, that is the one unforgivable sin in any society."

Obviously, that's said extremely tongue-in-cheek, not that the character of Scarlett O'Hara was exceptionally perceptive. But had she been left the mill by her husband, and immediately installed someone to manage it for her, it would have excited little comment — her insistence on active management, not to mention openly peddling lumber at work sites, to unsavory contractors, and carpetbaggers, was much more of an issue.

An interesting parallel here was when women began attending law school and medical school in the United States in more significant numbers — they were openly berated by their professors and fellow students for "taking a place a man, who needs to support a family, needed". Similarly, Scarlett O'Hara finds that the gentleman manager she'd hired from her wartime circle of acquaintance, Hugh Elsing, lacked "gumption" and eventually dismisses him from her employment entirely, even as she made excuses for Ashley Wilkes' similar failures. Aggressiveness is normally a reasonable thing to cultivate in business, but as early as the beginning of the novel, Mammy and her mother had been telling her, "You must be more gentle", "Men do not like forward girls" and so on.

Parents can only train their children in skills that they know themselves and anticipate that their children will need, and Scarlett's mother had been trained extensively in household management and the skills that ladies were considered to need — thrift, ability to persuade, cajole and command servants, nursing, piety, and so on.

The ability of women who have experience in managing a household to apply those skills to other situations can be quite good — witness medieval noblewomen, trained to defend their homes in sieges and provide for refugees from their lands in times of war — but Scarlett O'Hara never really showed much aptitude for them, as she always lived in other women's households (her mother's, her aunt's) until the sudden death of her mother, the sack of her family home, and a complete collapse of the currency, which would be a challenge for any young woman to begin with.

Blending the natural aggressiveness necessary for business with the gentility and manners of gentlemen for the ability to conduct business in such a way as to not alienate social acquaintances was not a feminine skillset, and in the novel, it's brought up many times that many of those women who took on more conventional work, like fine sewing, or painting china, or running a boarding house, were complete failures at the business. The only post-war businesswoman that was a success without torpedoing her own social position was Mrs. Merriweather and her bakery, and her long history of leadership in charitable ventures, social entertaining, and the distaff side of religious organizations undoubtedly helped her manage that minefield.

For more information, you can check out:

Steele, Jeffrey. "The Gender and Racial Politics of Mourning in Antebellum America" in An Emotional History of the United States, Peter Stearns and Jan Lewis, eds. New York, U.S.: New York University Press, 1998, pp. 91-108.

Wetherington, Mark. Plain Folk's Fight: The Civil War and Reconstruction in Piney Woods Georgia. Chapel Hill, U.S.: University of North Carolina Press, 2011.

Woods, Michael. Emotional and Sectional Conflict in the Antebellum United States. Cambridge, Eng.: Cambridge University Press, 2014.