Why do the North America versions have a different (read: lower) standard?
Cos your fuel is rubbish. The road haulage associations resist the reduction of sulphur in diesel, (which will cost almost nothing to do, few fractions of a cent per gallon is what i've heard) so you can't use common rail/high pressure diesel injection systems without MASSIVE expenditure in sulphur resistant hardware, or two completely separate fuel supply networks across the entire US. And the market is tiny. US car buyers won't buy diesels in the sort of volume to make it worth anyone investing.
Then you have the emissions standards in certain (most) states. Visible smoke, particulates and so on. Again massive investment for minimal return. (Though newer, Euro VI emissions standards are similar) Costs a packet to meet these standards too. The whole emissions standards thing needs completely tearing up and starting again in my opinion, much of the 30 year old technology that is still mandated has been superceeded, but can't be removed, cos its mandated. So we have to drag kilos of junk round to meet some arbitrary standard, that can be met (and probably exceeded) without the junk.
I believe VW/Audi and Mercedes are all attempting to crack the US diesel problem, all of them are shifting a handful of units, probably at a significant loss. I understand warranty is costing them a packet too.