I've been towing with a full sized Van since 1979 and they would be my first choice if capable. Except for about 6 years our two Van have been mainly used as tow or road trip vehicles and not needed or used as a typical daily driver. I love the Van on the road not for the people carrying aspect, but from the secure cargo space and it's inherent flexibility. IMO there are several reasons why you don't see more Vans towing trailers and some of the reasons might be these below:
1. They are limited to bumper pull and there are a lot of 5ers
2. For the most part Van are designed for large numbers of passengers so the demographics now days put them in the minority.
3. They typically are not "pretty" and are basically utilitarian BOXY vehicles and don't appeal to the more design oriented segment.
4. Unless you go to the conversion types which for the most part are produced on the 1/2 T platform they Van don't have the level of "luxury" you can find in things like SUVs.
5. They have a stigma of not being as maneuverable or parking friendly as most SUVs which while true to some extent it often is over emphasized. Plus unless you need the huge cargo and/or people capacity most families don't need those for "daily drivers" which is a requirement of many RVers.
6. They basically are not readily available in the 4x4 flavor
Larry