The short answer: The Giants have been sucking at every aspect of the game recently.
The longer answer: The Giants have been sucking at every aspect of the game recently. Most inexplicable is their defensive pass rush. They have proven pass rushers and, for some reason, have been incapable all season of actually pressuring the opposing quarterback. I really have no explanation for why Justin Tuck, Osi Unmenyiora, Matthias Kiwanuka, Black Fred, et al have been unable to put on any sort of pass rush. The rest of their defense is terrible at the moment. They're down to like their 5th string cornerback and 4th string inside linebackers. I understand why the secondary is not playing well, but I don't understand why the defensive line can't do them any favors. It's possible you could make an argument about since the secondary sucks so bad that other offenses can afford to keep more gies back in pass protection, but you couldn't do that on every play, nor does that really even make a lot of sense in terms of offensive scheming.
More explicable is their offense. It seems pretty clear to me at this juncture that the reason their offense has been so bad is because of the offensive line. Yes, it's true that the offensive line has been their cornerstone for the last 4 seasons, but I think it's precisely that reason that the line is falling apart now. They have played essentially every game in the last 4 seasons together. In that time, Kareem McKenzie has missed one game (this season, not coincidentally) and Diehl, O'Hara, Seubert, and Snee have missed none. The gies' reputation is that they pound the crap out of defenses and open up holes for the running game. They know where they're all going to be on pass protection and are therefore better at it than other teams. This is all well and good while they're all healthy, but that doesn't seem to be the case. There's a reason why every other offensive line in the league has been less durable and I think the wear and tear is finally catching up with them.
Brandon Jacobs is not doing anything differently, there just aren't any holes opened up. The Giants young, unproven receiving corps is has not really let them down this year. Rather, Eli has been pressured to the extent that he hasn't had time to throw to them. As much as the O-line has meant to this team, perhaps it's time to think about rejuvenating it heading into next season. O'Hara is in his 10th season, McKenzie and Seubert are both in their 9th, and Diehl and Snee are in their 7th and 6th, respectively. McKenzie in particular seems to be breaking down this season.
The Giants definitely are capable of making the playoffs this year, but they're gonna need to turn it around quickly. They have to without a doubt beat the Eagles in 2 weeks. They would be well served to beat the Cowboys next week, though it's not a 100% must win, it would be much better if they did. Assuming they beat the Iggles, they'd have to win at least 2 of their last 3 against Washington, Carolina, and a nothing to play for Minnesota. They're basically in a 3 way race with the Iggles and Packers for the 2 wild card spots right now, but at the moment they're on the outside looking in.
This is basically unrelated to their struggles, but I also just want to point out that I hate watching the G-Men come to the line with 20 seconds on the play clock and then watching Eli point out the mike, say "Omaha" 4 times, turn around and talk with the backs, send someone in motion, snap the ball with 1 second left on the play clock, and then just run off tackle. I understand that this is a fairly complicated game, much more so than all other professional sports, and that coordinating the movements of 22 people on the field is a big job. But seriously, isn't the run off tackle the simplest possible play you could be running? Block the hole open with the O-line, send a lead blocker in there, and hand the ball off. What's the deal with the rest of this?
Also unrelated to their struggles, and actually this point is helping them forward. I like how Eli is recognized now as the leader of the team. That was definitely not the case in the Tiki, Shockey years, and even through their Super Bowl run there were lingering questions. All of this lends to my thesis, "Fuck you, Tiki Barber, you prima donna, non-interview skill having, mid-season retiring, spotlight stealing hoe."