First, the POPMART TOUR WAS A SUCCESS. It is today still the 3rd highest grossing tour worldwide in history. Over 171 million dollars in Gross and 4 million fans in attendence for the 93 date tour. Only two Rolling Stone tours have done better worldwide.
In addition, the album sold 5.5 million copies in 1997 alone, enough to be one of the top 20 albums of the year worldwide.
Yes, its true attendence was off in the USA (perhaps by as much as 50% when compared to ZOO TV attendence figures) but the attendence they had in the USA was still enough to give them the biggest tour of the year in the USA in 1997. Some USA markets were thin, but new highs were set in such places as Chicago where U2 played 3 stadium shows for the first time in that market.
Bottom line, I believe U2 did the best they could, to market the material they had. It was obvious Europe would like this album and tour better than the USA. While the album sold 1.5 million in the USA, it sold nearly 3 million in Europe.
The only thing I can think of is perhaps releasing "Gone", I think that would of been a great single. Its the best song on the album and one of the shortest songs at around 4 minutes, perfect for radio programers.
Other than that, I think they did a fantastic job. People should realize that sometimes good material is not going to be as popular with the public in a certain country or region or the world in general, as it should be, regardless of the promotion.