Good question. One answer:
Why then would anyone buy an Illinois municipal bond, or accept a state contract that requires future payments, or move a business to the state, or keep a business in the state, or do anything else that required faith in the willingness or ability of the state to pay its bills? The only possible answer is that Illinois isn’t Greece; it’s Spain or Italy, an entity so big and important that its failure is inconceivable. When it hits the wall, Washington will have no choice but to step in and cover its unfunded pensions and teacher salaries and muni bond interest. In the same way that a Spanish bond is really a German bond because Germany has no choice but to make good on it, the big insolvent US states are wards of the central government.