1) LET IT GO, Queen Elsa and the True Love twist got mass media (both online and offline) abuzz, making Frozen a must see for families. Compare that to BH6 where the media interest has been 0.00000000000000000000000000000000000012
2) Disney musicals have immense potential to be huge due to their artistry. So when they are done in a way that captivates audience imagination through story, character and song like Frozen, TLK, Aladdin etc. they end up being immortal giants.
3) When it comes to movies, little girls are really under served when compared to little boys. Boys don't really need another superhero movie when they get 3-4 per year. Girls OTOH need movies directly targeted at them with strong female role models and themes pertaining to their interests.
4) The boy and his faithful thing concept has been done before in animated movies. And done very well. Princesses with any kind of psychological depth less so. No, Belle's Stockholm Syndrome doesn't count!
5) LET IT FUCKING GO! And all the other catchy and adorable songs that became viral sensations. BH6 had... Immortals.
With all these factors going for it, Frozen would have been humongous regardless of release date or competition.