Board Thread:Show-Related Questions and Answers/@comment-25974054-20150105192131/@comment-1894786-20150918201924

Stovers wrote: According to Regina, the Dark Curse scroll revealed that all fairytale chararcters would go back, cursed or not. Emma was exempt because she's the savior. Cruella and Ursula were never explained, but my theory is the magic from Lily's egg that kept them young also kept them here in our world. The egg "tricked" the curse into thinking they were natural-born citizens. No one outside of Storybrooke went back.

That's why in the episode itself, it's like seeing smoke float inside a closed candle. Because the magic was restricted to Storybrooke.