I think it lies with the fact that this is the first time in what, 20 or so years that Disney have had a new princess. I suppose Pocahontas was the last, prior to her, Belle, Ariel etc (except maybe Mulan? Not sure how well Mulan did). They're new, they're not the same old boring Cinderella, Snow White, Sleeping Beauty. Obviously I'm not saying Anna and Elsa are better than the old princesses, but that's what they are, old.
Anna and Elsa are the first princesses for this generation, so everyone is going to want to be dressing up as them.
We went to the UK version of Disney on Ice a few weeks ago. I think we are a bit behind you guys, ours is 'Celebrating 100 years of Magic' (which I can't understand why it's 100 years... nothing happened in 1914 from what I can tell?!). Anyway, every single little girl, bar probably one who I saw dressed as Snow White and one as Jessie, was either Anna or Elsa. But no Anna or Elsa appeared in the show! I was quite surprised, but I can't imagine how many kids went home disappointed that they didn't see them.
As the first princesses to come out of Disney in 20 or so years, I can see why they are milking it so much. Until they have another new princess, I've got a feeling this is how it will stay. They think the public want Anna and Elsa and I think they're probably right.
Well, apart from Tangled...