I imagine Apple provided a superior experience since they manage the App development frameworks on Mac / iOS. The flop must have been inventory. A lot of advertisers are now turning to exchanges (DoubleClick, AppNexus, OpenX...) for inventory whereas Apple forced you to use iAd Workbench [0].

[0] https://support.apple.com/en-us/HT202924

EDIT: The flop must have been demand, not inventory.