Re: Why do some great college QBs fail in the NFL?
There are so many what ifs and factors as to why certain players fail, it would take a lifetime to list them all. For example, what if Peyton Manning went to Detroit instead of the Colts? Would he still be successful?
What if Tim Couch went to a team with a solid offensive line and weapons around him plus having a few years to develop? Would he have been successful vs. being trust into the staring line up?
There are also players who shine in the NCAA but are complete busts in the NFL. Forget QB look at all the defensive players who have failed due to injuries or other problems (which is why I'm skeptical of Clowney).
To answer your question I think it comes down to (in no particular order):
1.) Not being placed in the right system.
2.) Being asked to do too much right away which hampers development.
3.) Injuries/overused in the college level and not having much left in the tank in the pros (true for many RBs).