1) Randy Shannon's last full class was full of busts. The sheer number of defensive recruits who didn't make it past their first two years here from that class is incredibly high.
2) Injuries have been brutally tough on this group since Golden took over. Curtis Porter is probably UM's best run stopper up front and he's missed the majority of the games during the Golden era. Ramon Buchanan had a chance to be an All ACC type player this year. That quickly ended. Denzel Perryman has missed a few games this season. The list goes on.
3) There hasn't been enough defensive difference makers in Golden's first two classes. Chick is one. Perryman is one. Eddie Johnson's one. Deon Bush is one. That's been it. In two years, when dealing with depth issues, all the injuries, and coming off Shannon's bad class in his last year, that isn't a big enough number.
If you're really breaking down the current kids who have been around for a few years, many weren't heavily recruited. Luther Robinson wasn't. Kelvin Cain wasn't. Tyrone Cornileus wasn't. Jimmy Gaines wasn't. The two starting safeties today weren't. USC and others out there let Vaughn Telemaque walk.
It's not like UM is giving up all these yards and points with a bunch of 4 and 5 star kids among the upperclassmen on defense. For example, UM is playing with 13 juniors and seniors. As per my thread on how the current players were recruited, lets take a look:
DE: Green 3, Cain 2
DT: Porter 3, Robinson 3, Smith 2
LB: Cornileus 3, Gaines 2
CB: McGee 4, Gunter 3
S: Telemaque 4, Rodgers 3, Highsmith 3, Swasey 2
That's a big list of defensive upperclassmen who are playing about the way they were recruited out of high school. Look at the Chicks, the Perrymans, the Deon Bushs, etc, and they were all heavily recruited (4 stars).
That's why landing uncommitted players like Matthew Thomas, Jaynard Bostwick, etc are so important to this year's recruiting class, and the same reason why UM fans should be very happy with the current commitments from players like Jamal Carter, Artie Burns, and Keith Bryant -- all of whom are national recruits.