I think it's that AR is so damned simple to figure out. Defenses now basically sell out on preventing him from running and bank on his inability to throw accurately/have touch and for him to make at least 1, if not more, critical mistakes (e.g., throwing a busted play pass to someone 5 yards behind the LOS for a TFL instead of throwing it away, or something like that horrendous FIRST DOWN duck of an INT, like he thought it was 3rd down and a deep INT was as good as a punt). If you rewatch the game (all of them actually), there are NUMEROUS horrible reads and throws by AR that I'm willing to bet Kitna would be completing, and a bunch of them are drive, if not game, changers. I could be horribly wrong, but I think had we put Kitna in after the first series, he completes more clutch throws with better reads and the rest of the offense steps up just enough (i.e., not banking on AR bailing things out) to make us far more competitive yesterday. At this point, and given how many times we see an open back coming out of our backfield, and given his poor choices trying to "go for it," I don't think it's that Billy won't hit backs out of the backfield, it's that AR won't throw to them until it is way too late and a TFL because AR thinks he'll be able to scramble out of a situation better than a back like Etienne can catch it and break up field (which is stupid). I was willing to give Billy the benefit of the doubt on that earlier in the season, but at this point, unless he is telling AR "do not ever dump to the back" as part of a design, AR not utilizing backs out of the backfield is on Billy, not AR.
Sorry this was a bit messy, I'm spit ballin' here!
It's been said enough, but while he is a great kid with amazing athletic talent, at least under two coaching staffs now, AR is NOT a good QB