Probably because it's in 3/4 time and Larry and Adam just aren't good enough to play it well enough w/o rehearsing the absolute shit out of it (see: the Streets intro and how it almost forced Eno to toss it during recording, after they couldn't play it right). That (= their quite-average rhythm section) along with Mr. Often Forgets His Own Lyrics, is a big reason they don't play setlists from a huge selection of songs.
It's not a tough song to play, at all.
Basic 4-chord progression, only two minor changes.
But Edge doesn't play a basic chord version of Acrobat, he plays something...a little odd. And another reason is that Edge probably couldn't remember how to play it. Of course, if he wanted to, he could re-learn it quite easily because it's not that difficult to figure out.
A last note on Adam and Larry...3/4 is the 2nd easiest to play for Western musicians, especially if the bass is all the same note (8th or 16th) and that is basically what Acrobat is (if you don't follow, think: timing of the notes played, all fast - all at the same pace, so you play 3/4 counting in your head -for instance, 1, 2, 3, 4, 5, 6 easy. It's tougher when you are playing a mix of different notes of different durations because it becomes tougher to count and hold the note, rather than play it as you count. Did that make sense?)
Window in the Skies is all in 3/4, I believe and hasn't been performed either. Has it?
Trip Through might be as well...? In any event, it's a plausible reason.