During Safety of flight, any airport is available
UGM is a very capable runway and has civilian aircraft fly there on a semi-regular basis. There are several government-contracted flights into there. Most every military airframe can land there as well. Biggest concern is getting the cuban airspace for the visual approach.
No doubt UA coordinated with Cuban officials and determined the best airfield to land. As for landing at EYW, based on fuel and cargo, might not have been doable. I'm sure the Captain and crew would not be excited about attempting a landing that where none have them have before. Been on on the FL 737 though do stop quickly, they had some distance until they were in the foam over-run area.
Again, easier to land at a international airport that has normalised relationships with everyone except the US, then land somewhere the runway is very short or flighing to the other side of the island for another airfield.