It’s simple. People of the world trust the USA treasury far more than they do from a communist country. The Chinese can offer great returns on investment but it is a huge risk. Would big (and small) foreign investors trust placing their life savings into the hands of the Chinese communist party? I say, HELL NO!
Uncle Sam has made a series of changes in the bills designed to make them more secure. Nork is shipping them with the new changes to SEA before we even see them in the banks. Officially the government banks do not accept hundred dollar bills but actually they do. I sent $2000 in $100s to a friend and protégé to pay off the rest of her home mortgage. It arrived and was taken to the bank to pay it off in $100s.
People prefer to save money with the bills because they are easy to stash and increase in value as the VND loses value compared to the dollar.