My good friend, and former safety at Jacksonville State, Jonathan Hagler said it best when describing cornerback play in the NFL:
It is hard to play corner, especially in the NFL. You get to play super aggressive in college and that just leads to struggles at the next level.
The position of cornerback is easily one of the most difficult to play in the NFL. That is why we see young corners struggle to adjust once they get to the NFL. They have to first get acclimated to the speed while also adjusting to stricter rules and more advanced offenses.