The reason that is becuase American car manufacturers have been horribly backwards with engine design. American manufacturers like cheap, with no regard to environmental cost. Turbocharged cars are complex and therefore are disregarded. They are stuck in a rut, American consumers hove your attitude to turbo motors, so manufacturer will make them for fear of not selling. The consumer can't buy them if noone sells a good turbo motor.
This however is changing. Get with the times. Engine downsizing is the trend within the industry to provide the power and meet emissions standard , you will find it in the vast majority of cars world wide. If not this generation, then the next generation of cars.
Ford is putting a 2.0L Turbo over it's V6 in the new Explorer. Similar power output with a boost in economy of aprox. 30%.
VW are replacing pretty much it's entire line with reduced capacity turbos. With the most extreme being the 1.4TSI putting out 150bhp. Audi as part of that group are doing it
Toyota are doing it
Volvo are doing it
Even Bentley are doing it in the next Continental GT.
Market insight from a consultancy website regarding turbos and downsizing to meet emissions standards.
A sort of technical pamphlet showing downsizing.
As I'm no longer at Uni I don't have access to the SAE papers archive. However there are a whole host of papers looking at cool stuff like variable vane turbos and direct injection to aid in downsizing.
I could go on, you can search the site on your own. There zillions of papers regarding this subject.
EDIT: You are doing a PhD iirc, your Uni is likely to have a subscription to the SAE paper database.
The advantage comes from the fact that you are running the turbo (ie compressing the air) with something you would otherwise just throw straight out the exhaust and not something you want to use. It's like adding an LP turbine to a power plant to take advantage of the energy still left.
I was talking about system efficiency not component efficiency.
Not only that, turbo cars a far easier to drive. As they have fat torque from the minute they come on boost. Leading to a wider power band.
Some of the things in this link are a bit overly basic, but it gives a good indication of how a turbo alters the power band and how downsizing an engine but using a turbo to match performance works.
If you want something with good mpg and emissions, it's time to get yourself a new downsized turbo then isn't it.