The full size body on frame vans were perfect for towing with a large family. As a kid, they were very popular tow vehicles and that's what my Dad used.
Where I think they fell out of favor is a combination of smaller families and pickups that could carry 4-6 people comfortably. In the 70's extended cab pickups were an oddity and even if you had one, they space in the back was not good for long trips. With the van you could easily take 8-12 and there was room to spread out and lie down on long runs (before the days of seat belt laws).
Now the big 4 door pickups can comfortably seat 6 adults and if you have young kids they have to be strapped into a car seat anyway, so now a pickup is much more viable for the typical 1-2 kid family even if they bring a friend, plus you have the pickup bed utility.