You really see it in the upper deck, 500 level on either side. These sections hold nearly 1,000 seats each. Most are completely empty with no single tix or any resale tix which indicates likely that these seats are not being sold yet.
Taylor Swift played to 56,047 in this set up with the stage at one end back in 2013. I'd estimate U2 has sold at best about 34,000 tickets for this show.
They obviously would prefer the field to be full before they start trying to sell the more expensive seats in the upper decks.
They'll at least make it to 40,000 by show time I think, which is equal to what they did on the ZOO TV tour in Pittsburgh back in 1992 at the Three Rivers stadium.
They did 55,823 on 360 and 27,785 on Popmart. The Elevation tour had an arena show that soldout at 14,863, but did not make use of the rear stage upper deck seating.
So that just goes to show that Pittsburgh is far from a reliable market for U2 with these wide swings in attendance for the various tours. I'd imagine also that 360 Pittsburgh was helped by the fact that there were no Ohio shows on 360 at all. Pittsburgh 360 was also the 2nd to the last show of the tour and the final show in the United States on that tour which also may have helped attendance.