From the NYT:

Economy and Crime Spur New Puerto Rican Exodus

 At the beginning of the 20th Century, the world was divided up into great empires, but the dominant political trend of the 20th Century was nationalism. After much turmoil, most of the empires are gone, and we live in a world of a couple of hundred independent countries. 

And, strange as it may seem from watching the 24-hour-news, the world is more peaceful and prosperous than ever. Sure, lots of former colonies remain badly run, but the general trend is toward slow improvement: after all, its their problem and they have incentives to get better at ruling themselves.

But self-rule is ideologically passe. Globalism is the default assumption: diversity, you know? Thus, the ongoing failure of imperialism and open borders in Puerto Rico is seldom portrayed as the ongoing failure of imperialism in Puerto Rico. The notion that maybe, after 116 years it's getting toward time for Puerto Rico to stand on its own two feet simply doesn't come up in 21st Century thinking.

Much of the problem is simply that we've replaced old-fashioned conceptual thinking with who-whom thinking. See, imperialism wasn't nice, and American elites believe in being nice to Puerto Ricans, so therefore it's not really imperialism.