That's exactly what's taking him so long. I have no doubt McD huddled with Beane and probably the Pegulas on the eve of the season, before trading McCarron and naming Peterman the starter, and convinced them that it was a good move for a variety of reason - Peterman had a solid preseason, Allen wasn't ready, the line was still gelling, the opening schedule is brutal and it would be damaging to Allen's career to put him in so quickly. Now if he goes back to them to wave the white flag so quickly, he loses face big time. He needs to pretend he's attending to some kind of deliberative process so that he doesn't look like a total flake.
It's that simple.