Puerto Rico has had a terrible economy over the past few years, with unemployment over 10%. As full U.S. Citizens, Puerto Ricans can easily move to greener economic pastures anywhere in the United States - and many have.
This. Puerto Rico is only slightly better than, say, the Dominican Republic. And unlike the nations surrounding it, there's really nothing stopping them from moving to the USA. Hence why there are more Puerto Ricans in New York City than in Puerto Rico.
Also I believe it would have 5 Congressional districts.