50 mpg in Europe is more like 35-40 mpg here... It makes sense they won't bring them over, well not really, but no one brings diesels over here... I think its to do with the fluctuation of diesel in North America. Our diesel fuel is derived from the same source as home heating oil so in the winter months when demand for heating oil spikes it ratchets the price of diesel fuel upwards...
I think the GLA in diesel format would sell quite well here, just not to the yuppie valley girl crowd queuing for the GLA and CLA...