I would think one thing that the Airstream has beyond simple box trailers is the ease of towing. They tend to be a bit lighter but their best attribute is the domed ends, there is just a whole lot less drag.
There's probably been a study done on how we like the looks or feel of something that gets iconic. Even if that object could be improved in function or style, there is a presumption of venerability or quality about certain designs. I don't look at the Airstreams or Avions as really that much different than other trailers. Of course, an old Swedish cowpoke like me is more concerned about the function of what I have than the looks of it.
IMO, the smaller Airstreams have good functionality for a certain camping style. They are easy to tow and are built more for the guy/gal that doesn't need all the bells and whistles of some other types. One size doesn't fit all.