I have often fancied teh idea of an Airstream caravan, but they have always been much heavier than similar sized UK caravans, and very pricey. It seems that nothing has changed much.
Historically I believe the original designer Wally Byam, was keen to make an aerodynamic shape, which is why the models have a strong resemblance to aircraft. In that regard he was successful, but its a shame he didn't also take on board the same aircraft traits for low weights.
Still it does mean the design is iconic, and as Alastair Clements points out it 's the uniqueness that is likely to attract buyers for that alone.
I am certain the design could be readily put on a weight reduction diet, using the aforementioned Air craft construction techniques. Some of the interior fittings could also be redone either in lighter material, or slimmer sections, without loosing the functionality.
Even in its home country of America, there is the begging of a trend towards more fuel efficient vehicles, and that will almost certainly involve reduced vehicle weights, so lighter caravans will need to follow if the RV sector wants to remain bouyant. I am sure that lighter US products would find more favour on this side of the pond.