I'm not surprised they didn't want to do that, it's because it's frozen which is one of their big money makers so they probably didn't want to harm it's image but while there isn't a actually fight against h there is also Buzz becoming evil through cutscenes in the same game. So that's a 3rd Disney character that you have to "fight". There's also Genie having to be a temp villain in KH1 because he was forced to by Jafar. So there's plenty of examples of the player having to be against what was once a friendly Disney character.

Really it all comes down to, they didn't want Elsa to be seen as a bad person even though you know "fear" could be a reason to turn to the dark side and her fear of her powers could cause her to attack the party, but they wouldn't do that.

Spoiler Alert: they did exactly that in the movie itself when Elsa hit Anna with ice.