Third leg Chicago took a couple of weeks to sell out. 'Nuff said.
I'll repeat what I've said numerous times before... Chicago is a major hub city for airlines and for those willing to drive. It's a given that U2 is not touring many cities during the first leg. As a result, the Saturday Chicago show (5/7) tix are in super demand because of the ppl that can fly/drive to Chicago from those areas that U2 is not touring. (May 7 GA's are going for $1000 on ebay.)
The other Chicago weeknight shows, by contrast, do not have as much demand. (GA's for weeknight shows are going for about $300.) In fact, seated tix for those "other" Chicago shows are going for far below face value. (I bought two $220 Sectin 333 tix for $50 total, including shipping on ebay!)
One variable is the DVD-shoot rumored for the Chicago shows. At this point, no one knows whether they will be shooting all four nights or just Saturday 5/7. At this point, the 5/7 show is an anomaly because of the Chicago's "hub city" designation, not because of the DVD-shoot. Otherwise, the demand would be through the roof for each Chicago show, not just the 5/7 show.
Not disrespecting the demand for the 5/7 Chicago show or even the Boston 5/28 show, but the MSG 5/21 show is in another galaxy. MSG 5/21 tix (even the worst pair of Section 400 seats) are selling at up to 4 times face value. No other show, not even Dublin or London, can boast such a mark-up owing to demand.
Just my 2 pence...