...Antigua and Barbados make more sense for connections. Then you can play the LIAT game.
They do, but fares to ANU can easily be £300 more than to SJU via NYC or MIA. Eagle also offer far more onward connections than LIAT or Winair + they can offer through booking.
I would imagine that most routes would need a certain amount of o&d to survive, but there must be more hotel beds in San Juan alone than in ANU&BGI together?