Federal Social Insurance programs, Social security, Unemployment, food stamps, and other transfer payments. These substitute for states inability to print their own money. and set exchange rates or control the flow of capital.
The Banking system is backstopped by the Federal Reserve not individual states. See Texas and California during the S&L crisis.
And the lack of tribal attitudes. Europeans think Greeks need to pay back all the money their government borrowed from French and German Banks. No one in the US really feels that way about Puerto Ricans.
As an aside, the states actually do have de facto currency issuing power as a function of their sovereignty and taxing authority. It's rarely exercised, but it has been done. For example when California failed to pass a budget a decade ago, it started paying with IOUs that it also accepted as payment for state tax liabilities[1]. They even strong-armed banks into accepting the IOUs as deposits. The total sum was nearly $3 billion. So the states can print money.
I think China in the last 15 years has taken the opposite tack, demanding regional governments run large deficits and loaning them the money. Which works because, Chinese leadership can give two shits about the books.