My guess is that between Cinderella and Aurora being golden blondes who both sometimes wear blue dresses, they wanted to make Elsa stand out physically. Maybe they went for platinum blonde but overshot to white, or maybe platinum is hard to process into a wig? I don't love it, especially when Elsa in person has super dark brows. I get that for stage makeup, you need strong brows (I'm a natural blonde and on stage, I have to supplement on the brows), but sometimes Elsa's brows are DARK...