I really think the great divide from the two disciplines of coaching football has to do with personality type. I can't remember where i heard it, but they say that the great NFL coaches are more of a CEO than a coach and great college coaches are great motivators and are extremely influential. In the NFL an eye for acquiring talent on the field and with coaches is paramount, from there you delegate out what needs to be done and are more or less the manager of the work-flow, hence the large number of coaches and large scouting network. In college you develop and motivate and you don't have to have quite the eye for refined talent, just raw talent, and the ability to get the most out of a player is massive importance. In the pros the player is responsible for their motivation, either that or their cut and away goes the money.
I really don't think that they can't be interchangeable, but what I think the crux of the situation is; is that you have successful coaches and unsuccessful coaches, and rarely do you see a coach cross over that has had a good deal of success whether it be in the NFL or college. There just isn't a whole lot of reasons to make the move, other than money. And when you're talking 4 or 5 million it's really not enough to make the change. That is unless you're a failed NFL'er like a Pete Carrol.
One of the problems and where it becomes difficult to evaluate is that you rarely see a coach take over a stable situation. I have no doubt that Tony Dungy or Mike Holmgren would be a great college coach, it's just that there is no reason for them to be and I'm not sure the do it yourself or get cut attitude would fly with a, lets be honest, more fragile teenage mentality of a college player. And if a college coach is good why would they want to step into a situation like Spurrier did? There just isn't that much of a upside.
All in all I think there are several guys on that would do well no matter what league they coached in. I think most of it has to do with the situation their handed, heck Bellichick was a failure in Cleveland. And I think you're seeing less crossing over because of that. Coaches just aren't eager to hop into a garbage situation just because it's the NFL. And good NFL coaches have no reason to step back into college.