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.
If there is one thing I miss about my beloved city, it's Mardi Gras. Caroline and I threw a big party every year. You'll always be my Mardi Gras queen, love.