Coaches have egos. Harsin will want to prove he can coach and that Auburn was just a bad fit, never got a fair shake, etc. Could he go back to Boise St? Arizona St? Another west coast school?
This is such an important factor I think most of us forget about. Money and ego drive a lot of these decisions. On both ends.
With regards to Harsin, I think everyone scratched their head at that hire. However, Any objective neutral can grasp that 21 games isn't near enough of a sample size under remotely normal circumstances. It wasn't a good fit on paper when he was hired, sure, but he ultimately good have turned the job down. He's getting $13.5 million plus money already earned for his troubles.
Arizona State and Colorado are surely better fits than Auburn. ASU is a bit of a mess and CU has their own shortcomings with facilities, etc. None of that might matter to Harsin if he gets a chance at either job.