The short version is that a lot of the debt is held by the people of PR themselves in form of bonds that were sold when the economy was good.
The reason it was artificially good is that for a long period it had great US tax benefits when compared with other US territories/states. During that time several corporstions (particularly pharma) had factories and jobs there. During that time there was a boom in bond sales - a lot of predatory practices that are reminiscent of the US housing bubble were done there, too. Once the tax loophole was closed, corporations left the island, jobs went to shit, the bonds could not be repaid, and all well, you get the picture. Except that in this case, "not paying debt", literally means not paying a huge chunk of all the life savings that people poured (arguably even patriotically) to their own bond system (see the parallels to the housing crisis?).
You could technically boost the enconomy there by reintroducing similar tax loopholes, but it would be a temporary fix, and there would be significant struggles in the Senate and Congress... Taxation without representation sucks big time...
Link to podcast:https://16683.mc.tritondigital.com/NPR_510315/media-session/...