The reason seems to be corruption. The debate is around the balance between intrinsic and extrinsic corruption, with good arguments on both sides.
Puerto Rico isn’t poor. But it is in a crisis. That discrepancy contains the problem and solutions.
Using GDP per capita (which is the wealth per person) is a very poor measure for the overall size of the economy.
This is akin to saying a SV silicon valley engineer who makes $250k a year should be able to pay his $2 million mortgage when he all of a sudden gets hit with $300k in medical expenses because he got hit by a bus while riding his scooter without a helmet - fuck it, he's wealthy. He just needs to budget better.
This is not a great ratio. But it’s not unmanageable.
The GDP per capita implies there are tax raises and service cuts somewhere which would be feasible, alongside cutting debt and liabilities. Much of the population being in poverty is an inequality, not resource scarcity, argument.
Additionally, the costs of goods and services is much higher than in the states. GDP per capita doesn't measure purchase power parity, and your dollars in PR don't go nearly as far as they do in the USA. Energy costs, water costs, shipping costs, are all much higher in other countries, and legislation (such as the Jones Act) that helps mainland USA hurts PR.
PR is trapped, and has a perpetual brain drain of people escaping. It has a negative population growth of -2% a year (in a heavily Catholic country), and GDP growth has been negative for 13 years in a row. More austerity measures are not going to help the situation.
Puerto Rico’s PPP GDP per capita has been stagnant since 2006 or so, but that’s true of Italy and Spain as well. (Italy has done worse over the last 15 years than Puerto Rico.) Its not a great situation, but doesn’t justify treating it like some developing nation.