For a long time, they enjoyed special low taxes as a commonwealth of the US (one of a very few). They thought that was great. That couldn't last, of course, and they have not been able to find ways to balance the budget since those low taxes ended.
A big part of the problem is that the government is in a kind of limbo. It's not a state, it's not an independent country. It's a territory, with some special provisions that they can't figure out how to work around.
In 2012, they held a vote: become a state or ask for independence. The result was more than 60% was okay with statehood, and more than 50% was okay with becoming an independent country. 75% of the people voted, apparently.
Then they submitted a request to the US Congress to start the processes to give them statehood: http://www.puertoricoreport.com/wp-content/uploads/2012/12/2012-concurrent-resolution.pdf
It's four years later, now. What has Congress been doing since then? Surely, in between
(I'm not sure why they would want to become a state now. Maybe we should require another vote, between statehood in this moribund political warzone we've become and full independence.)
Is Congress so willing to let an entire territory of the US become a population 3.4 million ghetto because they are hamstrung by their legal status as a territory?
What is the hold-up? Is it that so few speak English and so many speak Spanish and Congress isn't willing to let them be an independent country?
Shooting from the hip, if the US Congress is not willing, for whatever reason, to accept Puerto Rico as a proper state in the Union, we must ask them to become an Independent country and suggest, if they later still want to become a US state, to apply as an independent country.
Otherwise, we will continue to be the biggest contributor to the worsening conditions there.