Because some schools are willing to bend rules to get players cleared and our staff isn't willing to do that. Some schools also have more clout with their conferences than others. O$U is the Chosen One in the Big Ten. In all sports. The SEC wouldn't clear her. Not with her grades in the current shape they were in.
Think about it. She was going to wait until April to sign. Odds are if you're waiting until April to sign, unless you're a complete drama queen like Aja Wilson was, you're getting your grades in order. (That isn't a hypothetical guess with McCoy. This was the case with McCoy.) This doesn't happen very often because the vast majority of recruits don't have to worry about being cleared.
Should we assume that in the event Tenn signs anyone in April?