My understanding is that the Puerto Rican people and the Puerto Rican government are never synchronized long enough to allow application for Statehood to go forward. When the people want go forward with it, the government stalls. When the P.R. government proposes it for referendum, the people vote it down. They're never in synch. So, it becomes a never ending debate.
There is also the matter of space. The admission of new States would mean that the Senate chambers would need a redesign to make extra seats for the new Senators.
With respect to the discussion in the comments about California: since they are already a State, their people and government would also have to be in sync long enough to propose it to the U.S. Congress, then Congress would discuss it and debate it. The constitutional text is "New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress." As we have seen recently, legislators get voted out if they become at odds with those they represent.
Recall is that there was something in the agreement for Texas becoming a State that left them with the option to break their territory into five States. I don't recall my history studies well enough to know if that has already happened (back in the 1800;s) or if it is still an option for them. But Texas is Texas, given the way they are being treated, they may be the first to attempt to secede from the Union since the Civil War. The only good that would come of that would be that there are two open seats in the Senate and the Senate chambers would not need to be redesigned to admit Puerto Rico.