First, I don't think Campbell likes Iowa very much, if at all (although he and Kirk seemed to share a cordial enough handshake after Saturday's game).
Second good money at ISU and is likely due to receive some nice raises as he has some nice seasons and other schools come courting.
Third, he likes ISU and living in Ames with his wife and children. If they are happy he will feel good about that.
Fourth, the near future membership of the Big 12 puts his program in a position to win plenty of games.
Fifth, while the Big Ten and SEC TV deals will allow their schools to pay coaches more, people have been known to turn down higher paying jobs, especially if there is a lack of talent and the possibility of a fast hook involved. If Purdue offered him more money, why the hell would he go there?
Sixth, as mentioned above he probably has a short list of schools he would leave ISU for. Obviously, to date none of them have offered Campbell or his agent a job.