The SEC dominates because of DLine, and by extension, front seven play...
Pass defense. That's the common denominator. The national champ is invariably in the Top 10 in yards allowed per pass attempt. I think Auburn 2010 is the only recent exception. They had a strange defense that was often lazy but very talented and could rise to the occasion when Fairley was motivated. Overcoming the 24-0 deficit at Alabama was probably the most impressive comeback in recent college football history, given the caliber of the lead team.
I don't think it's complicated. The Big 12 forged a bubble screen identity and all but ignored pass defense. Those teams become dependent on cheap screens followed by equally lazy pump and go once the safeties rotate forward. That's half the offense in some cases. Oklahoma is often ranked #1 or nearby despite mediocre pass defense. Best of luck with that. The 2000 Oklahoma team that upset FSU in the Orange Bowl for the title had a terrific pass defense all season. Otherwise they've slacked, and been easy go-against when they made it that far. I'll never forget Bradford's stunned facial expression when Florida wiped out cheap bubble screens in vital short yardage plays. Blatantly unfair. All season those plays waltz uncontested into the end zone in Big 12 league play.
Leinart may have picked apart Texas in the Rose Bowl but that Longhorn team was #1 in the nation during the regular season in yards per attempt allowed. Texas under Mack Brown makes an annual attempt to play pass defense, although not always successful. The remainder of the league would just as soon ignore it. Sometimes the Big 12 defensive numbers are hilarious.
The Pac 10 (now 12) has a mixed identity. Further confused with hires like Mora, Leach and Rodriguez in the same year. The SEC was very fortunate to avoid USC a few times, notably 2008 when USC allowed a fantastic 4.7 yards per attempt. Not enough defense in the Pac 12 but athlete to athlete they are very capable, and certainly underrated as a conference by the typical sports fan.
The SEC features not only stalwart front sevens but aggressive defensive backs who ruthlessly deny the short junk. Last season the top four teams in the nation in yards per pass attempt allowed were from the SEC. It was Alabama, South Carolina, LSU and Georgia, in that order. Try to make a coincidence out of that. It's a necessary requirement at the top of that league, and subsequently the trump card variable in major bowls. You invariably see the opponent wilt when the SEC team simply refuses to allow underneath conversions on third down. Get the ball beyond the sticks or you're done.
I do agree that Andrew Luck or Kellen Moore would have been a fascinating test. Incredibly sophisticated schemes. Brandon Weeden's team carried so little defense I know how that one would have turned out. Stanford actually would have been a better test a year earlier, when Shane Skov was healthy.
BTW, when Miami was first ultra legit in 1983, we led the nation in yards per attempt defense. That was my hint to pay attention to that category. It's seldom let me down.