As far as I understand, there are basically 2 types of chest rewards. Some give global rewards, like item blueprints or side scenarios, which make sense to be tracked globally. But others contain items that are given to a specific character and have no global impact, which make sense to be limited to one per party to prevent them from being looted multiple times to give out to every character (and to add a bit of competition for who gets to loot the chest), but not necessarily to be global. Is there a specific reason for this?
One explanation may be that many items are unique, therefore there wouldn't be enough cards in the item deck to accommodate having the item exist in two different parties. Now, you could share that item between the two parties, but that sounds like not much fun.