Well, this issue is a bit more complicated than it might initially appear and gets into the fact that despite on-field success and fantastic ratings, the SEC was running 3-5 in terms of distributed revenue.
What appeared to have been a great deal for the SEC with ESPN, quickly turned sour as we saw other conferences ink comparable deals. For example, Texas if you combine the Big-12 payout with their network deal, Texas is still getting over 30 million annually. The Big 10 and Pac-12 payouts were nearing 30 million, while the SEC was stalled at 20, which closely mirrored the ACC And Big-12 payout.
Now, a lot of this has to do with the network deal. The Big 10 and the Pac-12 were able to pay out more because they had a network, and teams had to give up their tier 3 rights (Alabama made millions every year off their t3 rights), but the gap was still substantial if you consider that, to reiterate, the SEC had the best ratings.
But, the SEC has Texas A&M, they have Missouri, they have their network, even their being bound to ESPN for all eternity doesn't seem to be enough to stop their momentum now, they are going to get paid.