In the aftermath of the Snow Queen's spell, portals were opened up all over Storybrooke bringing in people from realms far and wide - across all universes (even the alternate ones). In a desperate attempt to protect the world at large from the influx of these newcomers, Regina sealed off Storybrooke permanently, making it impossible for anyone to leave the magical town.
i gottsa open a present early! santa told mommy and daddy it was okay if they gave me the present that was from them. i gotsed a kitty! his name is killer cuz he jumped through the wrapping paper to destroy it.