Part of it is because they have a bizarre system that puts them partly between independent and US state. And of course, another part is leftwing politics, since the Dems have been romping around PR for ages now.
They had a very good GOP governor a couple of terms ago who did a lot to stabilize their finances and start working on making it a productive place. It has everything it needs: not only is it a beautiful vacation spot, full of history, but most of its residents speak English and Spanish, and it also has the potential for small manufacturing, production of certain tropical crops (vanilla, coffee, etc.) and even perhaps being a Caribbean financial hub.
The thing that defeated the good governor, btw, was the presence of the civil service unions (yes, SEIU), since he wanted to reduce government employment and in fact make government jobs and benefits just like any other jobs, paying on the basis of skills and promoting on the basis of merit. The Dems don’t like that because probably about 50% of their constituency is composed of government employees or lives from government income.
Over half the population is on public assistance and the median IQ is 85. The only thing that could help PR is to deport its current inhabitants to the Dominican Republic and repopulate the island with Germans.