Sugar is hygroscopic, which means that it will attract moisture and absorb it back into itself. When you seal anything up in an airtight holder the moisture from the buttercream will get right back into the decorations that are on the cake. Ask me about what happened to the gumpaste flowers I put on a cake that was in a carrier overnight.
Also, if you put royal icing flowers on a buttercream cake they can absorb mopisture and oil from the cake and soften up. I once had a cake in the fridge that had royal icing flowers on it, and when I went to cut it the flowers had kept their shape, but they were as soft as buttercream flowers.
Cardboard boxes are better than plastic carriers for any kind of storage.