One reason, metal will last longer than plastic; more durable. I also don’t have a good place for them right now on the outside of my Tacoma. Using the Jerry cans on my rack was the best option for me right now.
Don’t get me wrong, rotopax are nice and I may mount 2, 3gl ones on the side of my canopy. But, I will have to build a mount for them so they can sit over the canopy glass. I’m researching what materials and parts I will need to make to that happen.