Why doesn't Stevie ever lower the key to the songs she can't sing anymore??

[OutsideTheRain]

For instance, Silver Springs....often didn't do it on certain night of the FM tour, and when she did it, struggled with it because it is too high for her. So why not lower the song to the key of A, instead of C?? She has done this with certain songs in the past and it has worked out nicely: ie Landslide, Beautiful Child, SDMHA, etc. Most people would never identify a difference.