Americans are lazy and, as a group, lack the simple skills required to use a wheel, a pedal and a lever all at the same time. Of course, there are one or two exceptions, but it pretty much holds water.
And the clutch pedal takes up valuable room that a stomach could fill.
Presumably a sensible answer is that the amount of congestion in the cities, coupled with the long, boring interstates makes changing gear more of a chore than a pleasure. In Europe we get to brake from 60mph to 30mph, heel&toeing down the gearbox every few seconds, so we can enjoy our driving (or we can mash the lever about and let the syncromesh do the 'blipping', but nobody who enjoys driving would do that, surely?).
And the massively cheap fuel prices meant that the efficiency drawbacks of a slushbox weren't ever thought about. And who would with a 6 litre V8 in a town car that can barely do 10mpg ever.