It's the same thing as what happened in Atlanta. Mike Vick failed primarily because he didn't have any weapons on the outside; he had scrubs like Roddy White, Michael Jenkins and Alge Crumpler catching balls. Then last year, with Vick out, the Atlanta offense couldn't move at all because the receivers just couldn't get open.
Then this offseason, Atlanta went out and got some bona fide weapons like Michael Jenkins and Roddy White to stretch the field for their rookie QB, and their passing offense predictably took off. Lesson learned: QBs are only as good as the deep threats they're given.