Love these kinds of threads. The answer to this question (which, I assume, is why the meltdown?) is pretty complicated, and delves into all aspects of the game, but as has been mentioned:
Reason #1: Bo Pelini is not a very good HEAD coach.
Assistant? Sure, one of the best, but head coaches of real significance simply don't EVER do some of the things he does. The abrasive, screaming at friends and foes on the sidelines, Leakgate, the way he treats 'his kids' in public. The whole package screams unprofessionalism, and is the antithesis of everything Osborne worked to build.
Reason #2: Scheme.
It's good vs. spread and zone read teams, but horrible against teams with a big O-line and a Langford, Ball, Gordon or Miller running behind. You can't 2-gap against Wisconsin's giants; they'll push you around all day. Pelini simply doesn't seem willing or able to change things up. Against teams like Wisconsin, you gotta shoot gaps with DL and blitz the other gaps; change up fronts, slant/stunt. I've coached for a number of years, and if you ask any kid what their favorite thing to do is, the number one answer will be 'blitz.' Gordon might have gotten us once or twice, but after we get a few TFL's, they would have went to the passing game, which is obviously not their strong suit. The fact Pelini didn't change and start filling gaps is a testament to the need for change. We beat (9) teams when we out-athlete them, but we won't out-athlete Wisconsin, OSU, MSU, or even McNeese State.
Reason #3: Effort
Pretty easy to see the drop-off in effort as a whole after Gordon leaped over Cooper's bone-headed dive tackle attempt, and we were STILL WINNING. Even after that, we were still in the game, and heads were dropped, guys weren't trying, and running lanes became runways. Wisconsin, down 14, didn't do that, because they have a coach that builds them up and teaches them how to face adversity instead of screaming in their face. After the 3rd or 4th long run, a good coach would have taken all 11 off the field and let them watch their hungrier back-ups do a better job than they were.
I've been in Pelini's camp since the beginning, but this is it. Nebraska football is no longer respected, and a lot of it can be laid at the feet of Mark Pelini. Say what you want about Callahan - giving up 70 points and whatnot, but under his tenure, Nebraska never became part of a National record that won't be broken any time soon. Or ever. Think about that - we'll always be known as the team that gave up 408 yards...to ONE GUY.