Older automatics in particular were less efficient and heavier. This mattered little when you put one in a 2-1/2 to 3 ton land yacht with a V-8 burning cheap fuel. The extra cost isn’t so much when you are already paying for a huge car with a huge engine, on easy credit terms.
It matters a lot when you have a small, light economical European car, with an engine so small that air conditioning was not the norm until the last couple of decades. Imagine a 1960’s VW beetle saddled with a 1960s automatic transmission…it wouldn’t have been able to get out of it’s own way, yet was passable transportation when equipped as sold.
Further, a lighter car takes a fair amount less finesse on the clutch than a big heavy vehicle, and the clutch pedal will require far less pressure as well. Especially as the normal manual transmission only had three forward gears, so first was pretty tall. A torque converter allowed a very low effective gear ratio, so made for better initial acceleration in the day.
The upshot is that until recent decades, automatics worked better in big cars, and manuals worked better in little cars. Further, American women started driving in large numbers far sooner than their European sisters, in a day when women were still expected to wear heels. The clutch pressure and travel needed for the large V-8 in the station-wagon that 60’s moms drove really was too much for many women to manage. American auto makers pretty well had to supply automatics if they wanted to sell more than one car to each household, while European makers were still dreaming that maybe 1 in 5 households might buy a single car. To sell more cars European makers had to make them cheaper. To sell more cars American makers had to make the sexier and easier to drive.
The main reason that Americans prefer automatics is that Americans prefer larger cars. And the reasons for that are pretty simple: Much lower fuel prices, affluence, easy credit, lack of tax penalties for larger heavier vehicles, vastly longer distances to be covered, and cities that were largely designed after cars were invented, so have wide streets and parking spaces, and a driving population that didn’t mostly live in cities anyway. There is a lot of truth in the saying that a European is someone who thinks 100 miles is a long distance, and an American is someone who thinks 100 years is a long time.
Now most of what has been said of modern automatics is true, but there is strong cultural inertia. Europeans still see automatic transmissions as frivolous luxury, and Americans still see a clutch as something for people who can’t afford a proper car.
A second reason is that American driving licenses require far less training than European standards. The typical American teenager is turned loose with perhaps 1/4 the training that a European would have, and that European would most likely be a decade older. There really isn’t time to learn how to drive a clutch in the time devoted to American driver education.
As for me I hate driving slush boxes, have never owned one, and find it wonderful that I can actually rent a car with a standard transmission in most of the rest of the world. Honestly, if you are not willing to drive a clutch, it is significantly harder to hire a car in Europe.