The decline of paid domestic workers happened kind of unevenly during the first half of the 20th century. By the 1950s, it was largely, though not entirely, obsolete, compared to its previous status as the primary form of employment for women. I'm going to focus on the beginning of the decline, and one of the major turning points in the history of "women's work" and housewifery in the United States-- the late nineteenth and early twentieth century. While I'd argue that this is definitely the beginning of the end (as I'll get into) WW2 was really when the number of domestic workers dropped dramatically.

The de-emphasis on domestic work was thanks in part to the rise of "home economics" as a field and the rise of a type of a new vision of nutrition and domestic work. Middle class women were increasingly targeted as consumers. Housework was no longer just positioned, in women's magazines and women's publications, as the rightful domain of a paid servant class, but as the work of housewives. Feeding the family, caring for household expenses, and keeping house was newly envisioned by social campaigners as professional work. The new housewife was a professional, ideally with training in home economics. tellingly, home economics programs, which catered towards the middle class, did not encourage women to become employed in domestic work, which was seen as below them. Instead, this "new" housewifery, which in fact encompassed the job of a domestic worker, was an aspirational ideal for young women, one that required proper training and management. Advertisers, social campaigners, and women's writers were increasingly articulating the "solution" to caring for the home as being the purview of middle class women. If women's housework is respectable and scientific--if feeding your family is a proper and difficult endeavor-- it should not be left to low wage employees.

This mindset arose with a chicken-and-egg relationship to "the servant problem," or the fact that the number of willing domestic workers shrunk at the turn of the 20th century. Did advertisers and social campaigners encourage women to do their own work because there were fewer workers, or were these ideas responsible for the decline in the field? As with many things, it's a bit of both. There were fewer workers, for several practical reasons: immigration restrictions decreased the presence of an urban underclass willing to perform the work for low wages (also, immigrants were increasingly americanized-- ie Irish women were no longer relegated exclusively to the ranks of domestic service as the century wore on), and other, better paid work was increasingly available (even long before WW2 broke out). Efforts at organizing domestic workers for better pay and better conditions almost inevitably failed because of the difficulty of communication, so there was no meaningful way to unionize or reform the industry. It just was what it was. When the depression hit, there was also no real market for domestic work, and WW2 ensured that there were many better jobs for women. And after the war, there was a mad dash to use wartime technology improvements for home goods. vacuum cleaners, dishwashers, frozen food, was all marketed to make women's work easier. These advancements were simply one final nail in the coffin. While all of these things are true, there was also unrest before and concurrent to the demographic change: typhoid Mary became a figure that represented the dangerous potential of a cook and letting someone of lower standing into your home.

Obviously, these changes didn't happen overnight. The decline in domestic work, came in part because of some very successful social messaging and branding that targeted middle class women, and that they in turn bought into (for various reasons, and to various ends). Consider, for example, the Ladies' Home Journal, which enjoyed a circulation of 1 million at the turn of the 20th century, and offered women advice that encouraged them to ditch the servants and respect their own work and their own place in the household. This messaging had two impacts: both encouraging and legitimizing women's place in the home, and in offering real validation of "women's work."

There's a lot of really awesome scholarship on domestic women and the "professionalization" of the housewife. I'd recommend Revolution at the Table, which deals specifically with questions of nutrition, which was more or less the most visible and essential part of a domestic worker's job. Also, Helen Zoe Veit's Modern Food, Moral Food, the dated but relevant survey of reasons of decline "Experts and Servants: The National Council on Household Employment and the Decline of Domestic Service in the Twentieth Century"