Well, the difference is that the band was providing a barrage of content via the stage show. As with ZooTV, there was a lot of choreographed material that prevented a lot of elasticity.
So when there's so much to absorb visually at and process (one could see several shows from ZooTV/Popmart and still see new things depending where you're looking), the setlists aren't as much of an issue.
When your stage show consists of a big screen for each band member? Yeah, you need to mix it up more and play from a larger well of songs.
They did manage to stuff Zooropa songs in the setlist mid-tour, yet chose to ignore everything pre-UF on the early legs on Zoo TV. I think the cause of the staticness on Popmart may be a part of consumer-obsessed culture they were taking shots at. A McSetlist for each town, if you will. The visuals aren't an obstacle to new material (see : 360).
U2 just isn't a jamming band that can toss out a new song when they feel like it, regardless of the incoming songs on the last two to three tours.
Realistically, what else can they pull out of the hat after Vertigo and 360 that would still surprise us ? Songs off Pop ? More B-sides in place of less played album songs ?