If Disney "truly" had the Formula for having a movie "strike lightning" like Frozen did, they would be be producing films in that "cookie cutter" formula, which in itself, would kill the popularity of the films.
Ultimately, as with any film, the heart of people being drawn to it is (a) great story (b) strong characters, actors, performances. Any filmmaker will say it comes down to story, story, story.
And with Animated films, you often have two additional elements of magic (c) great music; and (d) great art
----While I agree with others that Disney had the help of social media on this one--after all, the popularity of Let it Go even caught Disney off guard--the story was pretty solid, with compelling characters, music and art
--- the story starts with a hans christian andersen tale
--- I think Disney finally had a recent princess movie with likeable, adorable funny sidekicks in Olaf and Sven (reindeer)
--- "Let it Go" hit popularity with the music, as well as "Do you want to build a snowman??
--- two princesses instead of one.
--- Finally, a story that was more about two sisters than about getting the guy.
--- Cool graphics.... Elsa's ice dress is awesome, and the Ice Castle is spectacular.
As a movie, Frozen hit a lot of the right notes.... and Disney ran with it in terms of Merchandising, which is what Disney does best... I was in High School when Disney had a series of hits with the The Little Mermaid, Beauty & the Beast, and Aladdin followed by the Lion King a few years later... These movies had similar elements... great story, great music & art, compelling characters (especially in lovable, humorous sidekicks) powerful graphics (Under the Sea Production Number, Our Guest production number, Belle/Beast Ballroom, Cave of Wonders, Savannah scenes in Lion King) (Sebastian, Chip, Mrs. Potts, Cogsworth, Lumiere, Genie, Timone & Pumba, Rafiki, Zazu).
Also, the fact that Frozen was a princess movie, only adds to the madness... The Princess thing has really become a full on thing at Disney in more recent years, even more so since the early nineties with The Little Mermaid and Beauty and the Beast. When I visited the Parks in the late 70s/early 80s as a kid, it was about meeting Mickey & Minnie (and not through "meet and greet" locations), and certainly not about dressing up as a princess.
In fact, according to the Disney Parks blog, the Bibbidi Bobbidi Boutique didn't open until 2009! Its only recently that Disney has focused SO MUCH on the whole princess thing... Add a popular Princess movie and Whoah!
Add to that, that the fact that comparatively, Disney doesn't really have all that many princess films. Of Disney's many films (54 or 55, not counting the Pixars), there are relatively few "princesses" overall - not to mention the fact that they're spaced out over the years.
Snow White, 1937
Cinderella, 1950
Aurora, Sleeping Beauty, 1959
Ariel, The Little Mermaid, 1989
Belle, Beauty & the Beast, 1991
Jasmine, Aladdin, 1992
---------------------------Pocahontas, 1995 (not really a princess movie)
---------------------------Mulan, 1998 (not really a princess movie)
Giselle, Enchanted, 2007 (mostly live action, Amy Adams)
Tiana, The Princess and the Frog, 2009
Rapunzel, Tangled, 2010
Anna & Elsa, Frozen, 2013
So then, take the magical elements for a popular animated film (story, character, music, art), add in "let it go" and the soundtrack striking platinum; add Disney Merchandising, multiply against the centrality of "princesses" to the Disney Parks and overall merchandising strategy; and you have Frozen MADNESS.
Its not just DISNEY that does this...
----I remember a time in the 70s when all of my cousins had star wars toys, light sabers, pajamas, bedsheets. And now Star Wars is just as big as ever - ahem, star wars weekends at hollywood studios borders on craziness.
----Universal has similarly capitalized on the popularity of SHREK and princess fionna - What, there are 4 or 5 Shrek films now plus "Puss & Boots" ????
----I've seen tons of kids carrying around "minions"
--- I also seem to remember a time when everything was about cabbage patch dolls.
Different Movies are going to Come & Go in popularity. Disney is always aiming to find the next "timeless" classic. And anytime they strike gold, or maybe platinum with a film and its music, they're going to merchandise the heck out it (Both In and Outside of the parks)
So until the next princess movie hits popularity...
I guess we just have to "Let it go"