The problem is the U.S. uses MPG to measure fuel efficiency. MPG is not a measure of fuel efficiency. It's the inverse of fuel efficiency. Consequently, the bigger MPG gets, the less it matters. Consider a truck, SUV, sedan, econobox, hybrid, and research vehicle which have to drive 100 miles:
5 MPG truck = 20 gallons used
15 MPG SUV = 6.7 gallons used (10 MPG better, 13.3 gallons less than the truck)
25 MPG sedan = 4 gallons used (10 MPG better, 2.7 gallons less than the small SUV)
35 MPG econobox = 2.9 gallons used (10 MPG better, 1.1 gallons less than the sedan)
50 MPG hybrid = 2 gallons used (15 MPG better, 1 gallon less than the econobox)
100 MPG research vehicle = 1 gallon used (50 MPG better, 1 gallon less than the hybrid
See how there's a non-linear relationship between the MPG improvement and gallons saved? That's because MPG is the inverse of fuel economy. Passenger sedans and econoboxes may gain the most MPG from fuel-saving technologies like hybrid engines, but that's just an illusion created by MPG being the inverse of fuel economy. They're actually the worst place to be using these technologies. If we were really serious about saving fuel, we'd be working on putting hybrids into trucks and SUVs first. Basically every SUV you can turn into a hybrid is worth 2 econoboxes turned into a hybrid.
All this becomes crystal clear if you look at the first figure - gallons used per 100 miles - which is the proper units for fuel economy:
Truck = 20 gal/100 mi
SUV = 6.7 gal/100 mi
Sedan = 4 gal/100 mi
Econobox = 2.9 gal/100 mi
Hybrid = 2 gal/100 mi
Research vehicle = 1 gal/100 mi
You can see how once you reach about sedan-sized, you rapidly enter the point of diminishing returns, where even a tech which cuts fuel use in half saves very little fuel per 100 miles traveled. (Yes there are a lot more cars on the road than trucks. But if you developed a tech which cut fuel use in half, why outfit a million sedans with it, when you could outfit just 200,000 trucks or 250,000 buses with it and achieve the same fuel savings? Yes eventually you want to outfit all vehicles with it. But you should start by outfitting the vehicles where it will give you the most bang for the buck - the vehicles which use the most fuel per year.)
The rest of the world uses liters per 100 km, and so doesn't have this misperception that's prevalent in the U.S. Consequently, they've been working on improving cargo truck efficiency, instead of hybridizing tiny passenger vehicles so marketers can advertise a meaningless big number for MPG.