I think that U2 are no longer the MUST SEE band, especially in the UK and the US. The SOI/Apple "scandal" and the ripple effect backlash it caused are probably the main reasons. But there's a few other factors, like PRICE, the fact that U2 are not a current band with any current HITZ, and that so many people saw them on Vertigo and 360 and have crossed them off their bucket list.
However, if U2 are able to sell this many tickets at these prices, logic still dictates that if prices were around Vertigo prices, we'd have more tickets sold. The situation is not dire, people. Having a couple hundred $300 tickets still available is not a big deal in an arena that holds 18,000. U2 will eventually get asses in those seats, even if they have to discount them or give them away as corporate perks or something. Of the shows available, the only ones I would say may not sell out are LA 5, Chicago 4, Chicago 5, maybe one of the later Montreal shows, and the midweek Boston shows may have a few still available.
I think the only show where they might have to rearrange some seating and put up partitions and curtain off some sections is Chicago 5. I'll be at that show, so it'll be interesting to see how empty it is.
U2 are a classic rock band now. Most classic rock bands can't play 5 nights at the United Center, or 7 nights at Madison Square Garden. A band like Rush or Aerosmith would struggle to sell out 2 shows in those venues. Even the Rolling Stones didn't SELL OUT many of their US arena shows in 2013, despite what Billboard says about "sellouts". Also, the Stones, Aerosmith, Rush, etc don't do 360 shows, even in arenas. Springsteen is a very popular live act, but he plays to not quite sold out arenas all the time. SELLING OUT has always been very important to U2. I saw Springsteen in Columbus last year, and I think the official Billboard tally was 13,500 out of 16,000 something. The arena was in a 270 configuration, and you could see some empty rows in the upper bowl. But nobody really cared, Bruce didn't seem to care, and the show went on. If U2 thought there was a chance they would only sell 13,000 tickets in Columbus, which they probably would, they just wouldn't come. Even Elevation tour had some arenas not sold out, and some shows were only 270 degrees, with a curtain covering up the unsold seats behind the stage. The shows I saw in Columbus and Indianapolis were like that, but appeared sold out otherwise.