I think the primary reason is that 1) NFL is considered the top of the mountain and the majority of people with the pride of a HC don't 'want' to go back down (or regress) in their career path, if they have another choice. 2) The money an NFL coach gets is more than what the majority of college positions pay
Just to be clear, I'm not fighting for us to pluck someone from the NFL. If the choice is a HC with a shit ton of college experience (and success) Vs a HC with all NFL experience, I'd want the college guy. But I think the perfect or PRIME candidate would be someone with HC experience at both levels.
But if our choice ends up being someone with no HC experience (or poor HC experience at college level) Vs someone who only has NFL HC experience, but was even marginally successful in the NFL, then we'd be foolish to not take the NFL guy.
There are more cases of guys being average or slightly above average in the NFL then being rock stars in college, then there are of these same types bombing in college.
My choice 1 would be a guy with ample coordinator experience, who had some college HC experience, tried out the NFL and was decent but either didn't like it there or wasn't quite good enough to last and came back to college and is have solid consistent success.
Choice 2 would be guy who has only college HC experience and has shown solid consistent success.
Choice 3 would be a guy with only NFL HC experience and at a minimum above average success.
But absolutely, by no means, no more never been a HC guys...that's been 2 of our last 4 which has been our main problem.