It's the talent level.
Look at the guys playing on our offensive line. Daniels is the only NFL talent there. Some years we have had 2 or 3 NFL players on our line. And it isn't just coaching there - the guys aren't big enough for the spots they are playing (i.e. guards playing tackle).
The defensive line isn't any better. Who are the stars on our D-line? Ekakitie is a nice guy, but he isn't anything special as a football player. Anthony Nelson looks good but he has had trouble keeping contain. Matt Nelson is so slooooow. He looks like he is running in mud when he moves. The other starting D-tackle is so anonymous I can't even name him off the top of my head.
The receivers aren't running good enough routes and are too slow to get open. Pass protection has been horrid, especially on the left side, but the receivers other than MVB haven't shown they are B1G players either. A lot of people can run fast, and a lot of people can catch a football, but there aren't a lot of great receivers out there. We don't have anybody other than injured MVB who can do both, apparently. Scheel looks good but he doesn't get much PT, whether its attitude, poor practice effort, or whatever.
In the secondary, we have some duds too. Miles Taylor misses Jordan Lomax in the back because Lomax would play center-field, allowing Taylor to make the big hits that are what he is good at. Brandon Snyder looks like another slow, token white Iowa safety, in the same mold as Tanner Miller, John Lowdermilk, etc. Those guys make our cornerbacks look bad because of their ineptness. I rode Mabin really hard last year, especially when he put in that piss-poor performance against Illinois. He's been solid this year, and with his physical measurables, he should get a sniff at an NFL roster. King has been amazing in special teams, but he hasn't made those "big plays" that put him on the national radar last season.
On the whole, we have some stars who will play on Sundays, but not nearly enough of them to field a great team... and I don't blame coaching for that.