Oh, I'm not saying it *never* happens, but most people don't need to design their trailers to survive "regular" rollovers.
Making a bullet-proof trailer adds weight, which is something we constantly battle with, most of us driving downright mortal vehicles like Jeeps and Land Rovers.
As Martyn stated, being smart about your load will minimize the rollover damage in most scenarios, even with the current crop of "fragile" designs, which is why I've got a fairly obscene amount of Core-Trax linking the inside of mine:
