I would say for a series of choices made by the series authors.
The idea behind the show (at least in the first season) is the one of moving the fairy tale characters in a new world where magic is not known, which can be obtained by casting a curse.
Then, something magical is needed to break this curse. During the first season we learn that a major ingredient of the show is true love:
True Love is the strongest magic of all
as defined by Rumplestiltskin. Since it is so strong, this is the only thing that can break the curse (otherwise, the characters would be imprisoned in this world forever and without their memories).
Thus, two ingredients to run the show are needed:
- a witch that can cast the curse
- a couple that can, with their love, overcome the curse
Selecting the ingredients.
Now, true love in the classical sense is the one between the classical couples:
- Snow White and Prince Charming
- Belle and the Beast
- Princess Aurora and Prince Philip
(although we learn from the following seasons that true love can be found in new couples too, see [probably] Emma and Hook, Regina and Henry, Emma and Henry, Regina and Robin Hood. Still, the authors needed a "known" starting point, so they needed to use a known couple)
From these couples you can cross out those couples whose story do not involve an evil witch/sorcerer et similia that can cast the curse. So, we are left with a few of them (i.e., Cinderella&Prince, Snow White&Prince).
Why Snow White?
To me, Snow White is a good candidate for the following reasons:
- it involves a witch that is also a queen
- it involves heart ripping, from which you can develop the "heart control" and "heartless" topics (main topics during the series, see Cora, Rumplestiltskin, and the Knave of Hearts too for the Wonderland counterpart)
- it involves a lot of fairy tales characters, like the Huntsman and the dwarves
As a further reason, there could be (but I am not so sure) the fact that this was the first fairytale that was transposed as cartoon by Disney. It could be, since now in Season 4 we will have Frozen too (that comes from Disney), entailing a relationship between Disney and the show.
Adding an external factor.
And here comes who (to me) is the real main character: Emma.
As a third element (that I did not list before) there should be an external factor. Why is this? Well, if true love from the classical characters would suffice in breaking the curse, the situation would be solved in a single episode: the characters are sent in a new world, they recognize to be somehow in love, Snow White and Charming (or, better: Mary Margaret and David) have the first kiss in this new world, the curse is broken, see you all in the next season.
So, who could be related to a classical fairytale character (as Snow White), born in a fairytale land (as the Enchanted Forest), but external to the curse? A son/daughter of course, which somehow was sent across worlds to save him/her from the terrible curse, and that (as product of true love) can finally break the curse.