With the 50's you can do some uptempo Jazz, some Do Wop, rock and roll and they could have some fun with it. Manilow is all about soupy, syrupy ballads and these singers don't need any more of that stuff. A lot of people have gone home for choosing those songs.
I have to wonder if the artists that are appearing to sing on the elimination nights are there to have a chance to pimp their music.
That's the only reason. I haven't done the research, but it's entirely possible they also come from the same label that puts out the Idol stuff.