They're young and inexperienced. They still need to prove they can step up in those big situations. Maybe not so much Manningham because he had a great season last year, but the other two have had less experience in the NFL so now this is their chance to make a name for themselves.
I'm not discounting their talent, because I know they're all very talented. But sometimes that doesn't translate into the game, and JPP has a chance to start a lot more games this season so hopefully he steps up and has a remarkable season, which I believe he will.
Do not forget the shit-show this defense put on late in games last season, blowing huge leads and being unable to cover RBs out of the backfield or being mismatched vs TEs, and having their weaknesses exploited. They may have proven enough for the coaches, but they certainly haven't proven enough to the fans.
They cut guys on the OL, and their replacements need to prove a lot as well. Beatty could be a very good LT, but he is inexperienced. Now is his chance to step up and this opportunity by the horns and beat the shit out of it.