Automatics are much easier to drive for beginners and especially convenient in towns and cities. If anything, one would expect the situation to be reversed as long drives are more common in the US.
The simple answer people would give for not choosing automatic in the U.K. is "automatics are more expensive". What historical event made this not the case, or less relevant, in America?
Not to discourage further discussion, but this has been asked before, such as in this thread .
US auto manufacturers did a lot of work on automatic transmissions during World War II, and the M5 Stuart light tank had an automatic transmission of similar design to that used in cars.
Postwar, the car manufacturing cultures of the US and Europe/Britain were very different. US citizens emerged (generally more) wealthy, optimistic, and open to technological promise. Pre-war types were quickly replaced and the huge US domestic market was there for the exploitation, with consumers demanding an advanced vehicle.
By contrast, the British were saddled with rationing, and an automotive sector that was happy to go back to the 1930s. Britain was poor after the war, and this led to conservatism: people wanted a cheap car for utilitarian purposes, not a big V8 modelled on a supersonic jet. In Continental Europe this poverty was even more pronounced leading to very utilitarian cars like the 2CV and Fiat Bambino. Comparing cars from various countries emerging in the early 1950s shows this quite clearly.
This conservative mentality mitigated against risk, and the costs and 'futuristic' nature of the auto transmission made it less attractive.
Social changes saw US society- or at least its middle classes- move into suburbs which made cars more important, with women often being the driver.
Another factor could be the long, open nature of US roads which make shifting gears a less necessary and less rewarding task.