Apparently there either were no diesel wagon Audi Advants or VW Passat sold in the USA or there are just not any for sale.
There are Golfs and A3s diesel wagons.
The one thing is fuel mileage can be a little better. My mother got a Chevy Sonic sedan because the hatchback version had a worse fuel mileage rating.
Another reason could be safety. I knew a woman who died in a 25mpg single car accident because something in the cargo area bounced up and shot forward hitting her head and killing her. So having things locked in a trunk takes away the question of whether a cargo net is sufficient or not.
But I think the main reason is looks. People either like sedans, SUVs or pickups here. If it's not one of those three it can't be "cool."
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