Schatz isn't saying anything that we don't already know, namely that Big Ten running backs tend to be busts in the pros, but he does a nice job of quantifying the disparity. If he would extend his study farther back into the past, he'd find the same phenomenon occurring.
So why does the disparity exist? Personally, I think that SEC teams tend to play faster players, especially in the front seven, which allows their defenses to emphasize penetration into the backfield. SEC running backs then have to learn how to change directions right from the start of a play. Big Ten teams play slower defensive players and do not penetrate as much, so the runners are able to get a head of steam going through a hole. As a result, Big Ten runners tend to be in the Dayne/Anthony Thomas/Curtis Enis mold: big guys with good speed when they get going, but little ability to change directions or get to top speed quickly.
If you want an illustration, watch how Michigan defended Ron Dayne in the late 90s. Dayne tore up the Big Ten, but Michigan throttled him twice (handing Wisconsin two of their three losses in 98-99). UM had Ian Gold and Dhani Jones at ILB, two undersized backers who, not coincidentally, have been far more successful in the pros than UM's bigger run-stuffing ILBs like Jarret Irons, Erick Anderson, or Steve Morrison. Those backers shot into holes, making Dayne change direction at the start of a play. He was unable to do so, presaging his failure in the NFL.
The irony is that Big Ten coaches prefer bigger players because they think that bulk helps them stop the run, but as the teams in the SEC have shown, fast linebackers are the key. Sometimes, you can't rely on coaches to know what's in their own self-interest.
One other factor: Big Ten teams do a good job producing NFL-quality offensive linemen. They make life too easy on the runners, giving them huge holes through which to run. This was my theory for why Nebraska produced so few quality NFL runners during the 80s and 90s, although Ahman Green certainly showed that a good back can come from Big Red.
One other explanation, which I disagree with, is that Big Ten teams overuse their runners. If Big Ten backs carry the ball too many times, then one would expect that their career longevity would be less, but it doesn't explain why they are busts right from the outset. Also, there have been plenty of SEC runners who have carried the ball a lot in college and were fine in the pros. Herschel and Emmitt come to mind immediately. Ricky Williams, although not from the SEC, toted the ball a lot at Texas and that didn't stop him from being a very good NFL runner. Ron Dayne was going to be a bust in the pros regardless of how many times he got the ball at Wisconsin.