Oh boy, more of Merc's conspiracy theories.
To expand on what Griff said, ultra low sulpher diesel became law in Europe in 2005, available here in 2006, and law here in 2007, except for non-stationary/off-road/maritime use. So up until last year they couldn't use the emission controls US law requires on diesels now.
Oh, and their vehicles are lighter as they don't have to meet US crash standards.