College is much easier than pro. There's a lot more games in college and less money being bet creating much softer lines for NCAA action. The toughest college games are games between two top 25 teams, other than that, you can usually find 7-11 soft lines each week. There's not much parity in college so it tends to be a bit more predictable. The bowl games are pretty tough though.
NFL lines are the tightest of any sport. I usually go 60-65% for the year in college football. The NFL, on the other hand, I'm happy to be around 57-58% a year. My typical football (NFL & NCAA) season puts me 58%-62% so I can't complain with that return.
I'll take a shot at this... it sounds like an easy answer
Selection... Selection... Selection
With the NFL, there is a low number of games played each week. Therefore, finding good picks are easy in some respects (small # of teams to consider), but not many betting opportunities.
With College, there are much more games being played every weekend. Therefore, while the # of teams is high; it is easier to find those picks that are to your advantage.