Here are some stats on U2 concert attendance in St Louis and Kansas City since the Joshua Tree tour.
Joshua Tree Tour:
St. Louis Arena October 25, 1987 Attendance: 18,237(sellout)
Kansas City Kemper Arena October 26, 1987 Attendance:17,168(sellout)
ZOO TV TOUR:
St. Louis Busch Memorial Stadium September 20, 1992 Attendance:48,054(sellout)
Kansas City Arrowhead Stadium October 18, 1992 Attendance:37,867
POPMART TOUR:
Kansas City Arrowhead Stadium May 19, 1997 Attendance:23,709
St. Louis Busch Memorial Stadium November 1997 Attendance: 30,000(estimated)
ELEVATION TOUR:
Kansas City Kemper Arena November 27, 2001 Attendance: 14,500(estimated)
St. Louis Arena November 2001 Attendance: not reported yet.
I think the main reason Kansas did not sellout is because the show was announced only 2 weeks before it would happen. It took several months to get 23,000 fans into Arrowhead Stadium back in 1997 at much lower ticket prices.
The second reason of course is the close date in St. Louis. On POPMART these two shows were 6 months apart.
While its only 14,500 in attendance the GROSS is sure to top the 1,000,000 dollar mark. Not to many people turn down that kind of money which is why U2 went to Kansas City with only 2 weeks promotion.
Despite these factors I still expected a sellout because of U2 increased popularity. Even though it was not a sellout, they did come close, only 2,668 tickets unsold, and most of those behind the stage.
Just think about how much they Grossed in this small City in the midwest with a low cost of living!