Maybe it is because Jack, Kate and co. were never meant to leave the island. They were transported back to 77 to make amends and somehow change things back to how it was meant to be. If Claire and Aaron were supposed to leave the island (as in Desmond's vision) perhaps Sun and her unborn child were supposed to leave with her. Charlie sacrificed himself for Claire to be able to leave and Jin did the same for Sun. Somebody ( Jacob or Flocke, in the form of Christian) enticed Claire away from the group so that Claire did not leave. Perhaps the counter plan is for Lapidus to fly Sun and Claire away, to rejoin their children and the rest of our losties will remain on the island? Still not sure who is good or bad out of the two players of the game/contest. Time will tell!