This won't take long...
Oklahoma -- 1907
New Mexico -- 1912
Arizona -- 1912
Alaska -- 1959
Hawaii -- 1959
Puerto Rico has been the property of the U.S. of part of A. since 1898. Three States were added since the Spanish-American War to form the contiguous 48. The addition of non-contiguous states happened once, for Alaska and Hawaii, 61 years after Puerto Rico became war booty.
To complete the nationalization and permanence of the territorial limits in geopolitical terms, essentially, to make everybody belong to the same government, it made perfect sense to add Oklahoma (in the central portion of the country, for you statehooders) and Arizona and New Mexico in the southwest (on the border with Mexico, for you commonwealthers.)
Adding Alaska, a landmass equal to roughly 25% of the entire "lower 48", and Hawaii, a Pacific sinecure, also made sense, as both were long-held territories where American interests were ripe for development. In the case of Alaska, it turned into federal reserves; in Hawaii, tourism-related development.
So, if--IIIIIFFFFF--Puerto Rico were to ever become a State djqwgfcbkfegkgdsklipkx (sorry, I fell on My keyboard laughing), it seems that post-1898 has shown what the basic criteria are for being invited:
1) Political expediency, i.e., the forging of a potentially stronger political unit, or
2) Economic enhancement.
Do We satisfy either or both of them?
By 1959, We were already being plucked by American interests and as they say in My neck of the woods: Why buy the cow when you can get the milk for free? (Usually reserved for discussing pre-marital sex, but given the sad reality of Our economy, the intercourse analogy holds up well, doncha think?) There's no doubt the U.S. of part of A. makes tons of money off of Us, so statehood is really not an economic enhancement for them and in fact, is widely considered either a chance to carry freeloaders or a bottomless pit of economic rehabilitation. Usually both.
So what about political expediency? Well, We're not Anglo-Saxons. Or Protestants. Or native English speakers. Or descendants of the same parts of Europe that they are. Toss out all that as a unifying factor. We are way south of the border, and unlike Alaska, We are tiny with no natural resources and unlike Hawaii, We don't occupy a strategic location under U.S. interests. We did in 1940; We didn't by 1945.
If Puerto Rico--in the eyes of its political owners--were deemed worthy of statehood, it would have been decided between 1912 and 1945, when Our economy was nonexistent, the potential for American sacking--er, investment--was very high, Our strategic location could have been considered vital, Our population small enough (and in their eyes, stupid enough) to absorb and the "lower 48" were a unitary done deal. That it didn't happen then means it ain't gonna happen at all. Ever.
The sooner a good chunk of Us get that through Our heads the sooner We can focus on other matters, like finding a new path to Our development that doesn't involve begging a jaded suitor with what's left of Our decrepit charms. What's past is past, but as long as We continue acting that what's past should happen now, We will continue to look back with muddled eyes instead of forward with sharp ones.
The Jenius Has Spoken.