My guess is that it was to save her voice.
However, I wonder if it has something to do with saving money. In the original, didn't she sing that song to Len Cariou? But I don't believe that's him sitting there. Perhaps it was cheaper to pay a chorus member to sit there and cheaper to pay her to lip synch.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.