This is what I gathered from reading an Oklahoma message board:
Oklahoma doesn't want to come to the SEC, they see us as inferrior to them even though the University ranks right in the middle of academics of all SEC schools. Also, they think by playing in the PAC that recruiting California is going to make them a powerhouse program that will not be able to be matched by anyone else in the country. They think if the go to the SEC, even if they win a national championship it wouldn't really have an effect on recruiting because they would still just be another SEC team.