The biggest reason is revealed in episode 10 of season 2, The Cricket Game :
Snow White is roaming the execution site at night when she meets
Rumplestiltskin. Inquiring as to why he is there, he replies that he
has been disappointed by the lack of an execution. Snow White says
that she will not apologize for sparing the queen's life.
Rumplestiltskin says that Regina changing will not be easy. He asks
how she is going to accomplish this task. She says she does not even
know if it is possible. Rumplestiltskin offers to make a deal. A deal
to truly see if the Evil Queen can change.
Snow White: All you need... (opens cell) Is someone to help you let her out.
Evil Queen: What are you doing?
Snow White: I'm letting the woman who saved my life go. This is a chance to start fresh, Regina. To leave the evil behind in this cell.
Evil Queen: (walks to the cell door) Just like that?
Snow White: Just like that.
Evil Queen: You make change sound so easy. (Whilst walking out of her cell, Regina pins Snow White to the wall and chokes her. Snow White gets out a knife, but Regina grabs it away from her. ) Did you really think this would protect you? Since I can't use magic, I can think of no better way than to kill you with the blade you had meant for me. Goodbye, Snow White.
Snow White: No... (Evil Queen stabs her)
Evil Queen: Yes...
Snow White: (firmly-with sadness) No.
Evil Queen: That's impossible.
Snow White: No, that's magic.
(Prince Charming enters with guards)
Prince Charming: Rumplestiltskin. He took one of your hairs from this, and used it to fashion a protection spell.
Evil Queen: No...
Prince Charming: Now there's nothing you can do in this land to hurt Snow or me. You're powerless against us.
Evil Queen: You tricked me.
Snow White: It wasn't a trick. It was a test – one that I had truly hoped you'd pass.
Prince Charming: We wanted to give you a chance to change, Regina.
Snow White: Regina, you are banished. Banished to live alone with your misery.
Prince Charming: As long as you're alive in this world, you can't hurt us.
Snow White: You saved my life once, and now I've saved yours. So we're even. And if you ever try to hurt anyone in my kingdom again, I will kill you.
Evil Queen: What's so special about today?
Rumplestiltskin: Snow White and Prince Charming's wedding, of course. Didn't you get an invitation? Me neither. Still, nice to be able to see them declare their true love in front of their entire kingdom. A happy ending after all.
Evil Queen: And, because of you, there's nothing I can do to stop it. No way to harm them in this land ever again.
Rumplestiltskin: Yes. Yes, I suppose that's true... In this land.
Evil Queen: (curious) What?
Rumplestiltskin: The deal I made was explicit. You can never harm them in "this" land. Now, were you to bring them to another land... Well... (the Queen smiles)
Rumplestiltskin: Told you I was your friend. (disappears in a puff of red smoke)
Evil Queen: Father? Bring my carriage. I have a wedding to get to.
So with the protection spell active, chances are Snow White and Prince Charming would have survived the curse if left behind. By bringing them with her to Storybrooke, Regina could watch the two interact without ever knowing who they truly were- "forever" apart.