EDIT: I kinda doubt it was the result of 2005 Ozzfest, which didn't seem that succesful, at least according to Wiki
"At Iron Maiden's last Ozzfest performance, on 20 August 2005 at the Hyundai Pavilion at Glen Helen in San Bernardino, California, several negative events took place. During the first song, several members of the crowd, brought on by Sharon Osbourne, bombarded the British metal band Iron Maiden with eggs, bottle caps and ice after vocalist Bruce Dickinson allegedly ridiculed Ozzy's need for a teleprompter during his performances, as well as the Osbourne family's ventures into reality television (The Osbournes, Battle for Ozzfest).[12] During three of Iron Maiden's songs, the P.A. system was switched off, cutting power to Dickinson's microphone and then to all the band's instruments. During the concert, Bruce Dickinson can be heard accusing the festival's organizers of deliberately cutting off the band's power. On Iron Maiden's departure, Sharon Osbourne came on stage to make a few statements, telling the audience that she "absolutely loved Iron Maiden" but thought that Dickinson is a "prick."
I remember people at the time saying Maiden stole the show wherever they played, and that rather than a supporting slot on the Ozzfest bill they should have done a headlining tour on their own. There was even some speculation that, apart from Bruce's behaviour, this triggered jealousy on Sharon's part that resulted in Eggfest. Whether or not that is true is for you to decide.
The reason why 2008 was a success in the US was because it fully catered to the Play Classics! crowd. I really don't think there's more of a mystery to it. Maiden's name was out there again thanks to a number of tours and this time they promised to play all the hits and didn't threaten to play anything recent, so that's that.