The reason why there's going to be no Stadium shows in North America is a) because of today's prices (an average $90 per ticket for this tour). And even if U2 dropped their average ticket price to $60, they would only be able to fill or sellout shows in around 12 markets:
Edmonton - Commonwealth Stadium (55,000)
Toronto - Skydome (60,000)
Montreal - Olympic Stadium (65,000)...or two shows at Parc De Drapeau (70,000)
Foxboro, MA (Boston area) - Gillette Stadium (100,000...2 shows)...or three shows at Fenway Park (105,000)
New York City/NJ (area) - Giants Stadium (165,000...3 shows)
Philadelphia - Lincoln Financial Field (65,000)
Landover, MD (Washington, DC area) - FedEx Field (60,000)
Miami - Pro Player Stadium (45,000)
Los Angeles - Dodger Stadium (100,000...2 shows)
Oakland/San Francisco (area) - SBC Park (65,000...2 shows)
Chicago - Soldier Field (135,000...3 shows)
And b) because there's already arena shows on the first leg in most of their largest markets and hence would cut into their overall draw in these areas. If they did do a few Stadium shows on the third leg this year and dropped their ticket prices to an average of $60, then the above markets that already have scheduled arena shows would look like this:
New York City/NJ - Giants Stadium (100,000...2 shows)
Boston - Fenway Park (35-40,000)
Landover, MD (Washington, DC area) - FedEx Field (55,000)
Los Angeles - Dodger Stadium (50,000)
Oakland/San Francisco (area) - SBC Park (35,000)
Chicago - Soldier Field (55,000)
...with the Denver, Phoenix, Seattle, Vancouver, San Diego & Philadelphia shows being half full.
Also, today's shows sold out almost exactly as I thought they would, considering the $90 average price (which is basically only an 8% increase from the Elevation tour, minus inflation). The only market that is selling faster than I though it would, is San Diego. But I figured since it's opening night, that many LA and Vegas (epecially since there's no scheduled show there so far) fans will make the trip down to San Diego to see the show.
U2's being very careful not to price themselves out of the market this year...