I think another thing is lack of options.
Theoretically it would be best to buy a vehicle sized for what you need (example: Prius v) and not a smaller one you have to permanently attach cargo boxes to. But there has been a pretty big void between small +50mpg cars and -20mpg cargo haulers. And although that's changing, that's not going to help the used car market for some time.
I find it hard to believe that adding a cargo box at worst hurts fuel mileage by only 0.65mpg and at best increases it by 0.2mpg.
