Why do Europeans buy cars that get better mileage than do US consumers? I think most of the difference is due to the price-elasticity of demand and tax policy.
Fuel costs a great deal more in Europe than it does in the US, and most of that difference is due to taxes. One reason that diesel engines are so popular in Europe is that in most countries diesel fuel is not taxed as heavily as gasoline.
Here in the US, diesel is often more expensive than gasoline. If a diesel engine costs $2000 more than a gasoline engine, and the fuel is 25% more expensive, then it is hard for a US consumer to make an economic case for a diesel engine.
IMHO, the best way for the US to increase the mileage of the fleet is to increase fuel taxes, rather than mandating fleet economy standards. It's easy to change what people want by manipulating the cost; it is harder to get people to buy what they don't want. Unfortunately, both the Democratic and Republican parties here in the US are allergic to raising fuel taxes.
The recent fuel price rises in the US have significantly changed vehicle demand in the US. Sales of traditional body-on-frame SUVs and full-size pickups are down significantly. Toyota's brand new full-size pickup truck already has $2000 on the hood to move it out the door.
It is going to take a while for the US fleet mileage to change, simply because people are not going to replace their vehicles overnight. In addition, the emission regulations are creating significant technical challenges for diesel engines.
But things are changing in the US.