San Juan is a miserable city, but Nuuk isn't exactly Copenhagen, either. Puerto Rico at least has practical ways to get off the island.
Yeah. Normally I'm a sucker for whatever the wilder, colder, less urban option is in these polls, so Greenland should in theory be a lock for me, but it's insanely remote and hard to get to. If were living in Nuuk, Greenland's largest city, and wanted to visit family in the US, I'd have to:
1. Fly from Nuuk to Kangerlussuaq (Nuuk's airport is too short for transatlantic flights, so Kangerlussuaq is the hub)
2. Fly from Kangerlussuaq across the Atlantic to Copenhagen
3. Fly back across the Atlantic again to finally get to the US (and Copenhagen has very limited direct flights to the US, so it would almost certainly be at least a two-hop flight)
Puerto Rico is just a quick hop across the Gulf of Mexico.
You can literally fly to Iceland from Nuuk directly, and Iceland is a very built hub for trans-Atlantic stops both ways.
Yes, you don't have to backtrack all the way to Copenhagen (it is much more common to get to Greenland via Iceland), but the limited, cancellation-prone flights to Iceland from Nuuk are 3 hours on a Dash 8 Classic.