Yeah, I always wonder the same thing! Another song I wish she would play is "Private Radio". As far as I know, she only played it once in 2004 or 2005 and she was concerned about the echoes of the chorus and she joked that people would think she was lipsynching (or something like that lol), but I'm not really sure if this is the reason why she never performs it. Oh, and there's also "She Floats"! This one too, afaIk was only performed once in a show I have the audio. It was the penultimate song, after that she ended with "Twilight". It's a 2002 show. That live version of "She Floats" gives me chills, the intro is way better than the album version and I wonder if anyone knows if the studio version was supposed to be like that in the beggining (sorry for running out of the topic!).
Well, I guess you can all be right on your guessings. I just wish that on the new tour she'd do at least one medley with older songs, that way she could sing more songs we love in the usual 11/12 songs set.