Fantastic work by Jimmy Kempski (two links in two days). He timed every throw by Michael Vick and Nick Foles last year:
The Eagles have a very dynamic set of skill position players. When your QBs are holding the football for 2.87 seconds on average, you're essentially asking the QB to do a large portion of the work in your offense. That is not ideal when you have one aging QB that is in steep decline and another that was a rookie 3rd round pick. The quicker you can get the ball out of your QB's hands and into the hands of the guys who should be making plays, the better the offense will be.
Definitely check out his full breakdown. You can even tell when the Eagles decided it was time to change things up for Vick (as confirmed by All-22 tape).