On a world scale the U.S. has the lowest gas prices on the planet, the lowest tax rates and the best tax breaks of any industrialized country. So in a nutshell, fuel consumption isn't as important as it is everywhere else, and horsepower is king. It's the only market where you hear the majority of the consumer base clamouring for bigger engines and more horsepower (thus the demise of the 2.5L X3 in the U.S. market when in comparison it's the biggest seller elsewhere).

Where I live (Calgary, Canada) you can't spit without hitting an X5, but they are almost all the 3.0L variety. I've only ever seen one 4.8 on the road and very few 4.4 or 4.6's.

As an American who lives in Canada, I've seen both sides of this issue (and I sure miss my low gas prices and low taxes).