The Broadway has no balcony, there's only a mezzanine. True, it's a little far from the stage but I've sat there multiple times and the view has never been partial. I can't imagine anything about the set that could make a view obstructed from the center of the rear mezzanine (maybe only an extremely low false proscenium which Cinderella doesn't have.)
Thus, I think it's purely financial which seems puzzling for a show with such a strong advance. Perhaps they didn't want to take a huge risk so they didn't sell the rear mezz for previews but will once the show opens. It would be strange to leave those seats empty if they can sell them...
Scratch and claw for every day you're worth!
Make them drag you screaming from life, keep dreaming
You'll live forever here on earth.