Puerto Rico, like the Dominican Republic and many other island countries in and around the Caribbean is not a state. There are only 50, the 48 contiguous states plus Alaska and Hawaii.
The Dominican Republic is mired in their own internal difficulties which many do not want mandated by the United States.  Though tens of thousands have lost citizenship in recent years due to changes in the law, the administration stands firm behind their choices.
As for Puerto Rico, in the most recent election, not only did the majority decline statehood, they voted out the governor who pushed for it. The United States government is not pushing for it either.  Citizens of the tiny island can vote and have the protection of the US without being restricted by American federal interference. Change is unlikely.