Because Disney usually try to tread very carefully with sequels, and they’re always careful not to disappoint any of their core fans and audience. At the same time, they rarely if ever play it safely, doing the same thing over and over again.
If a sequel is coming, then it will arrive with various new developments to the overall formula, and it will advance the story of “Frozen” significantly. It will of course preserve the elements that made the original so interesting in the first place, but it will go far beyond that as well.
That’s the main reason fans have been so excited – they know the importance of the very fact that Disney have greenlit a sequel to their favorite film in the first place.
So, even though “Frozen 2” is supposedly not coming out anytime soon – 2018 is rumored as the most likely time for its release – it’s definitely going to be an amazing entry into an already very successful franchise (or soon-to-be franchise anyway), and it might easily overtake the success of the original, solidifying itself in a position as one of the most iconic productions in animated cinema in a very long time. Once the more interesting rumors start coming out, we should see clearly.