How many coaches do you know that stick around after they've accepted a job before a bowl game has occurred?
Theres a number of reasons that coaches leave once they've accepted the job. I'm thinking primarily that it's a gamble if you stay. Yeah, you can possibly win and add that to your resume, or you could end up losing and looking bad. On the other hand, you can add "12-0 and top defense in 2012" to your resume and that looks pretty nice, not to mention it's guaranteed.
Anyway, been outta town this weekend, pretty sad to see this news. It's certainly not unexpected, and Coach Diaco more than deserves a chance to be a HC...but I was kinda hoping ND would have at least bottled him up for another year or two by paying him big to stay. This could just be a ploy for more cash, which again would be warranted for his performance, but it just makes me cringe thinking of Diaco leaving already.