Its not a conspiracy.
There are many things in play here:
1. Diesel is a cheater fuel
2. US EPA Fuel Economy substandard are more realistic.
3. European emission standards are dirty and different, preventing cars from being imported.
To expand:
1. Diesel is more dense in pollution, oil and energy and $$$ per gallon, so it has to have ~12% more MPG to be equivalent efficiency. That is why I call it a cheater fuel because it tricks consumers if they don't understand the chemistry. I've been looking forward to some diesels beating hybrids, but they don't if you drive any city.
2. Back in 2008, the EPA lowered MPG ratings because drivers complained that they could never meet those MPGs. So, EPA ratings are changed to be more realistic. That has do with the fact that people speed and don't drive efficiently or other factors like roof racks or heavy loads. Today, if you don't get your EPA ratings, than your driving is more extreme than the the EPA, your auto maker lied to you, or your driving is just too fast or you driving needs adjustment. Ford, Hyundai, Kia and more have been in hot water lately because they fibbed on their fuel economy rating. Yes, the EPA does not test every car, so the automakers get to give there own numbers.
Euro fuel economy duty cycles are weak. They are much slower and not realistic for the US. There are also stories of fake test with taped door seems, thin oils, drastic weight reduction, ect.
3. The reason that Euro diesels do not come over to the US is mostly dirty fuel standards. Americans drive way too much and Americans are fat and sick because of it. Cut back the driving and the emission standards dont have to be so stick. Euro has dense cities with good transit. I blame the commuters. Sprawl just makes taxes go up and more government. The sad thing is that the Euro and the US EPA standards could align in the future and it would be good for everybody, but I have looked into it and there is no plan for that.
Euro cars are also practical, but gutless by today's standards. People don't use all the power in there cars to drive, so what is the point.