It’s evolved from a small pouch as we think of a pouch to a suitcase or briefcase to a set of baggage or trunk to boxes stored inside a shipping container to the whole shipping container to multiple shipping containers.
Diplomats are allowed to transport their cars, so I think that’s likely when shipping containers were first classified under ‘diplomatic pouch’ security.
The industry of container ships and shipping containers is not so old, a few decades.
Its set of diplomatic privileges applied to whatever conveyance is required to move your personal notes, cars, or the embassy furniture.