My impression is that the PR’s economy took a hit when they ended the tax breaks given to industries that located there. Being that they are 500 miles out into the ocean, there are added costs to any enterprise located there, which the tax breaks sought to offset. NAFTA, for example, was viewed as hurting PR as it encouraged companies to look south of the border rather than out into the Atlantic.
Heavy industry would probably never locate there, but light industry did before and could again. Of course, we still have the problem of getting companies to stop fleeing to China, which is a lot further away than San Juan.
Still... there may be other things they can do to turn things around besides just restructuring the debt.
US and European manufactures would rather go to China than PR for the same reason they prefer China to Africa, WORK ETHIC.