Concussions could be one thing. But I think there’s enough prestige & cachet in the QB position, not to mention salary potential, that concussion risks ought not to limit the candidate pool overly much.

Personally, I think there are some major factors:

• Seems like there are comparatively few colleges that run pro-style offenses, so too many of rookie QBs only know how to succeed in pass-heavy offenses against inferior defenses, and can’t handle the transition to running the different scheme against opponents who are all pretty good.
• Success in the QB position requires skill in an odd combination of areas: memory, pattern recognition, charisma, super fast decision making, ability to hit a moving target with a ball over a line of tall angry men out to hit you, enough ability to scramble to keep defenses honest, strong enough to take repeated painful blows & keep ticking.

I don’t think anything could realistically be done on the second point, but on the first one it would help if more colleges ran offensive schemes typical of what’s in the NFL.

Problem is, schemes in the NFL are great for situations where teams are closely matched. But college teams often have wide disparities in physical talent that can only be offset through creative schemes.