I blame U2's lack of improvisational skills on Edge actually. Since the band has so many different guitar sounds, its impossible for him to randomly float to different songs here and there because of equipment they might need for said song. That and Bono is terrible at remembering lyrics.
For the world's biggest band this is about the lamest excuse one could come up with.
1.) The Edge's guitar sounds are handled by his techs and himself, and it would be totally feasible to program and save all his sounds ever with the technology at hand.
2.)Bono has been working with a teleprompter since at least ZooTV, so his lyrics are available at all times. I don't think many people would find it degrading even if bono held the lyric sheet in his hand while singing either.
3.) When they bring out unexpected, unrehearsed songs, it's Edge and Bono most of the time, so it raises the suspicion it's actually Adam who doesn't know the songs by heart, since Larry could be excused for needing the break from the demanding job he's doing all night. Or maybe he doesn't deserve the excuse. Of course there's also the fact they are working with backing tracks, and that is a delicate and less flexible construction, than playing straightforward rock music. Besides it could be they've been working too long and almost exclusively with a click-driven set of backing tracks that is prohibitive towards improvisation, and might have made them afraid to work without it.
The major thing is unfortunately they aren't musicians, they can play what they practice long enough, and they've never practiced enough to know their entire catalogue, or even to be confident playing under-rehearsed material, so they just don't.
Let's not excuse this with perfectionism, please. It's another trait we are looking for. :-)