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.
With the 4th of July coming up, please keep in mind that there are some here that could be affected by fireworks due to PTSD from war. On behalf of those people, thank you in advance. You'll be saving us some flashbacks.