Good question - I've always wondered the same. I've been towing with a van for 20 years and prefer over a PU/SUV.
I usually remove all the seats so I have a big open space and can place a motorcycle and 2 bicycles inside - all locked up with and protected from the weather. If I want, I can use a roof rack and take a kayak with me as well.
No sure why vans are not as popular but it's ideal for me. I currently have a Chevy 3500 towing a 27 foot trailer - A dream towing vehicle, good wheelbase length, great brakes, strong engine/transmission. I currently get about 13mpg towing.
LarryJM makes some good points about vans being "utilitarian" and "boxy" but it really works for us.
Gerry