It's because gasoline is cheaper here than it is in almost any other highly-developed country. At $4 a gallon, gasoline/diesel is only barely more expensive than it was 30 years ago (adjusted for inflation). So why should behaviors change? I notice it took a sustained gas price of over $4 a gallon here before people started adding the price of their commutes into the price of those McMansions in far-flung "suburbs" -- and then housing there took a dive unrelated to the bubble. In Europe, they pay up to twice what we pay per gallon. Granted, a significant chunk of that is general taxation that funds mass transit and those evil safety-net programs. But you can bet if gasoline suddenly cost $7-8 a gallon here, people would be abandoning SUVs by the sides of roads, leaving the keys in the ignition.
And, unfortunately, most Americans have bought into the fictions that 1) you can't drive anything smaller than an SUV to be safe on American roads; and 2) any vehicle that can't go from a dead stop to 60 mph in six seconds or less is a rolling highway death trap. Fuel mileage would be much higher if people didn't buy in to that nonsense.