Automatic transmission is more expensive to buy and more costly on fuel so we in the UK just find it too expensive to buy and run a car fitted with automatic transmission. I also suspect that thwe Americans like their gadgets more than we do. As it so happens I went over to automatic 4 years ago and would never go back to manual now.
Early automatic gearboxes were simple three speed affairs, which is OK if you've got a huge motor under the hood with lots of spare torque, but a smaller, less torquey engine really needs more gears to work to the maximum efficiency.
US cars have big, low revving engines, whereas in Europe we've traditionally had smaller, higher revving engines.
I suppose the nearest we had in the UK to an American car in the 50's & 60's is the Roll Royce, which used a Buick V8 designed some time in the early Iron Age. Rolls also used the Buick 3-speed column shift auto gear box. They're still using that engine, but now have an 8-speed auto if I recall correctly...
There's also a fair amount of power loss absorbed in the automatic transmission. In a manual gearbox with top gear being a 1:1 throughput (no reduction or step up), there is a minimal power loss, I believe auto boxes absorb something like 10/15% an engine's power output.
That, together with the popularity of the American big capacity 5/7lt V8s (because gas was too cheap for the carefree/wasteful US public), meant that they didn't care about the terrible fuel consumption or losing 20/30 bhp just driving the transmission units.
Whereas broke post war Europe still had swinging ration restrictions and high duty on fuel that meant we had to go down the economy first driving route.