I think Frozen caught them off guard. If you look at a map and check out the stuff that's been closed, the Premier Theatre that the Frozen Sing along is in is part of an area that I figure is going to be for the expansion. Here's an image I drew up in a different site to denote the "expansion" area:
http://imgur.com/fgnajMs. Note that Lghts, Motors, Action is marked for removal, which I am predicting to be announced as closing at the end of August, September at worst. (This is based on Lights, motors, Action being previously involved in the runDisney Wine & Dine Half Marathon pre-race taste, and this year it has been moved to Indiana Jones, along with the Tower of Terror 10-Miler not being run this year. I suspect demolition will be started promptly after it is announced to be closed. Otherwise, why stop runners from being allowed in there after hours).
I think Frozen caught them off guard, they acted quickly to incorporate it into the park, and it forced them to delay as it turned out the sing along became more popular than expected. Suddenly a theatre which would have been torn down earlier is now seeing tons of action. I don't think they're dragging their feet, I think they're biding their time until they can give Frozen a permanent home, and keep the fans happy. I suspect once they close down Lights, Motors, Action that'll be the "starter pistol" round that triggers the complete reconstruction of the area marked in the image above.