They have so many damn hit songs they could swap in and out. Tunes that they've either already played so far, or are at least familiar enough for the general U2 concert audience to know and enjoy.
I Will Follow, Out Of Control, The Electric Co.
Gloria
Sunday Bloody Sunday, New Year's Day, 40
Pride, Bad
Where the Streets Have No Name, I Still Haven't Found What I'm Looking For, With Or Without You, Bullet the Blue Sky, In God's Country
Desire, Angel Of Harlem, When Love Comes to Town, All I Want Is You
Even Better Than the Real Thing, One, Until the End of the World, The Fly, Mysterious Ways
Numb, Stay (Faraway, So Close), The Wanderer
Discotheque
Beautiful Day, Stuck In a Moment You Can't Get Out Of, Elevation
Vertigo, City of Blinding Lights, Miracle Drug
Get on Your Boots, Magnificent
That's 30 from albums alone, and not adding everything from Songs Of Innocence or non-album stuff like Invisible and Sweetest Thing, etc. They could be interchanging a lot of these and still give the "New stuff plus lots of Hits!!!" feel they seem to love going for. I think Bono (and maybe the rest of the guys) just love having a theme/feel/whatever to a tour so much that they don't give themselves any wiggle room to play around because everything is so damn calculated.