The SEC just doesnt have as many good coaches now. Some of them left because of him, but mostly schools have just been making bad hires.
A lot of good coaches retired, were fired, or otherwise have left the conference over the past several years:
Phil Fulmer
Urban Meyer
Steve Spurrier
Mark Richt
Les Miles
James Franklin
Bobby Petrino (yes a scumbag, but the guy can coach)
Hugh Freeze (yes he cheated, but I think you're blind if you don't think he's a great coach)
Four of the guys in that list won titles at some point in their careers. Richt almost did. Even Gene Chizik, who I don't think is a great coach, won a national title but had to be fired a couple years later.
In basically every case, their immediate successor was a notably inferior candidate or just turned out badly. Hopefully, there has been an infusion of some better coaching within the conference with Pruitt at Tennessee, Jimbo at A&M, and Mullen at Florida (but he moved intra-conference).
Also, you want to figure out the rise of the Big 10 relative to the SEC, look no further than coaching. Urban, Franklin, Harbaugh, Dantonio. Urban and Franklin left the SEC, so the SEC's loss was the Big 10's direct gain in those instances. Pat Fitzgerald I think is a pretty good coach. Ferentz is overrated but a pillar of stability and very rarely has a truly bad team.