There’s been a lot of articles published over the past few years about the massive economic problems plaguing Puerto Rico. This one in Slate is the most recent.
Basically, the economy has been terrible for a long time. It’s so bad, in fact, that the population has shrunk by over 5% since 2000. Incomes are very low, unemployment is very high and nearly half the population is under the US poverty line. When it’s easier just to move to the mainland to find better opportunities than to stay, many people will choose to stay. The economic trouble and the fact that PR is far poorer than any state are two things (though certainly not the only potential issues) that I suspect will prevent the island from becoming a state if the people ever decide to push for that. It also means that independence isn’t a serious option and won’t be for many years. So, even though few people actually want the status quo (outright independence is unpopular too: the population is roughly evenly split between statehood and being a territory with greater autonomy than now), it seems that that is what Puerto Rico will be stuck with for the foreseeable future.