Okay, first of all, I hate the attraction's name. "Frozen Ever After"? What does that even MEAN?
Second, what, exactly, is the story supposed to be? I didn't see a story. It's just... See Olaf and Sven. See the Head Troll. See Elsa singing. See Marshmallow. Anna doesn't even appear until the final scene. The Pooh ride at Disneyland has a clearer story than this, and that can essentially be summed up as, "The weather's lousy, somebody put something weird in Pooh's honey, and now it's his birthday"!
Admittedly, I do like the idea of having Marshmallow make an appearance... And having the queue pass through Wandering Oaken's.
Because the song is popular and they needed to shoehorn it in. Really, it's not even that good of a song. I prefer THIS version over Idina Menzel's: