It's probably mostly standardization, I'd guess. I think a more sensible design would be to have the middle of the trailer low to the ground with a floor hump

It's probably mostly standardization, I'd guess. I think a more sensible design would be to have the middle of the trailer low to the ground with a floor hump above the rear wheel well and the front where it hitches on to the truck, but that would make loading and unloading awkward. One could imagine having a side door instead of or in addition to a back door, and a lower loading dock. I get that a rectanglar storage compartment is easier to deal with in a lot of ways, but it just seems ridiculous to have a box riding four feet off the ground, with the wheels causing as much atmospheric drag as they possibly can. This seems like a solution you'd arrive at if you think energy is practically free, and wasting energy has no consequence. It also seems unnecessarily top-heavy, in addition to the safety problems mentioned in the article (and the underride guards proposed to mitigate the safety issue would most likely create even more wind resistance).