For the same reason singers like Liza, Judy and others wound up with rasping relics of their previous voices. They either took care and maintained their voices or abused them into oblivion. Lupone obviously knows what she's doing, just like many opera singers who have maintained - for the most part - their vocal qualities over the years. When you're young, proper technique and maintenance isn't that important, but when one ages the neglect becomes all too obvious.