Unless you're Notre Dame, Stanford, Vandy, or a handful of other elite academic schools, if you get the test score (17, right?) you're gonna get in
I don't buy the academic argument at all.
IMO, the real difference is that the SEC has always put the best athletes on the defensive side of the ball...And yes, there is more speed on defense in the SEC than there generally is in other conferences...Obviously, the SEC doesn't really stand out offensively, with the possible exception of Arkansas.
So basically, the SEC has more talent on the defensive side than anywhere else. Every NC winner the SEC has put forth has been dominant defensively, while shutting down what is usually one of the best offenses in the nation.