Here is a quote from WIllie's diary:
.... We’re getting into a situation now where we have so many great songs worked up that the show is potentially too long. The Edge always laughs and says “Ah, its great value for money!” but the truth is that if even a great show is too long it throws out the dynamics of the whole event. For U2 something in the region of two hours ten minutes seems to work the best. We’ve touched two and a half hours on occasion and even though the crowd is delighted, you can feel them getting tired. That said, I have been pretty determined to get “Bad” into the main set for this leg, so we decided to add it in, even though we couldn’t find another song to drop to make way for it..........