I'm just spitballing here, but, what if CMC has not been contacted by ND? I mean it was pretty obvious that USC was going after him. According to ND though, they haven't reached out to anyone at this point. Would you want CMC to come out publicly and say, "I'm not taking the ND job?" when he possibly hasn't even been contacted about it yet, let alone offered it?
Let us not forget, coaching college football isn't necessarily the most stable job on the planet. Even if you are happy as hell where you are, you can't afford to burn any bridges along the way. In reality, CMC is only 2 or 3 seasons in a row of not making a bowl and then having to start looking for another job. I would think any smart coach knows it may anger the fans, but with whom his job security matters, it's probably best to be known as the guy who keeps his mouth shut until he has something concrete to say.