The question is also why they decided to alter the set at all. Perhaps it was the presence of relatives (including Ali) who made the guys want to do something special on the second night. Another reason might me the audience. I have seen U2 (and this tour) in a couple of different (european) cities, and I must admit, that my hometown of Copenhagen has been the audience that definitely responded most disappointing to the new stuff. The atmosphere was undoubtedly good both nights, but it was thanks to the "classics". At least a dosen of people went pass me to go to the bar/toilets during acrobat at night one, and songs from both the I&E-records did not seem to get people that excited at all. I doubt that most people even knew the new songs.
Maybe I'm a little hard on my fellow countrymen. But I don't think the appreciated the new stuff as they should, and I'm excited to see, if the changes in the set yesterday will last during the rest of the tour, or if the band actually had to adapt to a tough crowd in Copenhagen.