Crash rating wouldn't matter because it's not going to overrule payload capacity.
Not overrule, but they are trying to satisfy conflicting goals, which means greater compromise. Bean counters and lawyers have more to say about final design than you might think.
Fully boxed frames are kind of new on heavy duty pickups, and "overlanding" is more popular as well.
The forces that a frame might be subjected to are all over the map, depending on use. Open C frames have been "reliable enough" for a long time with hard mounted service bodies. Try using one as an overlander though, and you'll probably have a cracked frame where the bed attachment terminates before long. It may be a surprising to some, but the payload number is mostly determined by scaling from past experience... the number of warranty claims, lawsuits, bad press and pissed off customers, etc. Sure there is extensive modeling, but when you are offroad the *possible* forces that the frame might be subjected to are extreme. The question is, how many customers are going to do things that cause a failure, and is dealing with the aftermath worth beefing up the frame?... which will cost more, and interfere with crash standards.
