Reason 1 and 3 are totally baloney. European cars were as much boxy as the Americans and Japaneses in the same era and technology have nothing to do with enabling new curvy design either. If you look back to the 50s/60s, cars from Lincoln, Cadillac, Ford, Bentley, Mercedes, etc were not only curvy but there were all kinda of crazy complex shapes.

I think people's preference just got tired of the overly designed styles up until the late 60s. I would point to Cadillac as the one that started the boxy cars trend. They are still boxy/edgy today, although much more stylized. I love the newer Cadis design actually, they are so different than anything on the roads.