I've been doing some family history research recently, and it appears that a number of my ancestors had servants/maids (not slaves) who lived in the house and assisted with domestic duties. None of these ancestors were exceptionally wealthy. For example, my paternal grandmother grew up with a servant in her household, and her dad earned about $5,000 a year (equivalent to about $90,000 a year today) according to the 1940 Census.
Today only the extremely wealthy have live-in servants. What accounts for this difference?
I'm giving this answer mostly based on two books by the same historian, Frank Trentmann:
From their titles alone, you may be able to guess that Trentmann would give a very goods-based answer.
There's the obvious answer that domestic tasks became far less time consuming numerous thanks to inventions during the interwar years. To give just a few examples, the first electric dishwasher was created in 1929, fridges from 1913 (and had leaps in technology in 1920s with the development of freon to aid cooling), and electric irons from 1926 (though it took until the 1930s for any commercial success). All of these were the preserve of the upper-middle classes before WW2, as the upper class retained live-in servants, and the lower-middle class and working classes performed some combination of hired help and own work. Increasingly available home plumbing and electrification after WWII (particularly in Europe due to the need to rebuild) helped these technologies spread to most people, reducing the need for domestic servants.
Conversely, Trentmann also argues that the want for domestic servants was actually reducing during the 19th Century. The Industrial Revolution was at its height, allowing for a gluttony of consumerism of manufactured goods from Europe, and cheap availability of rarer goods from the world. The abolition of slavery in Europe in the early 1800s also played a minor role, as the overt 'use' of people became slightly tainted (exploitation of Europe's empires remained just dandy though...) As a result, many people, particularly the middle-class, began to shift how they wished to exhibit their wealth. Whilst in previous centuries, a key form of wealth display was by employing others to work for you, it became far more fashionable to show off your refined taste in goods such as fine china and houseware, or high quality clothes and furnishings. One example that sticks in my mind is an American woman in the early 1800s who proudly served her houseguests dinner on delicate china while the wind whistled through the windows; Trentmann's point is that there was a fixation on indulging on certain parts of one's life, even at the obvious cost to others (same as today to be fair).
Taken together, changes in what was considered the best way to exhibit wealth in the 1800s, and technological improvements both becoming widely available and desirable in themselves, meant that the demand for domestic servants had drastically reduced by the mid-20th Century.
Hope this helps!