I think the biggest difference between the NFL and college is the rah rah factor. College kids are motivated by coaches who inspire them in football and in life. In some ways the coach is like another father figure in their lives, guiding them from recruitment to graduation. Coaches who can get close to their players do well in college.
In the NFL players are more about business. They play where they can make the most money, and even though winning and camaraderie is important it's not like college where you go through four years with the same class of guys. In the NFL, coaches who are more businesslike tend to do better.
Some coaches can do both, but most coaches to transition from college to the NFL fail. Even Pete Carroll failed several times before his current success with Seattle. IMO, it's always a gamble for an NFL team to hire a college coach.