I'd be interested to hear your idea, because I don't have the slightest clue...
Just spitballing on why McNabb and not Culpepper or Vick, but I think it might go all the way back to the day he was drafted. The Philly fans were livid that the Eagles took him over Ricky Williams. Voila, instant victim for the PC media to feel sorry for.
I think he is a decent QB, but Vick is a better athlete(though I'm not convinced he'll ever be more than a RB who happens to throw the ball), and Culpepper is probably the better pure passer, but neither one plays in one of the largest media markets and gets to make soup commercials with his mom.