Urban Meyer's run-first, spread option offense calls upon a quarterback to carry a significant load when it comes to the quarterback zone reads.
This works very well in the collegiate ranks, where mismatches in speed, strength, etc., all make it very possible for a quarterback to run through the opposition. A big fellow the likes of a Tim Tebow, for example, can easily clear the trenches, and take on 190 lb safeties for even more yardage, while still being able to match an opposing linebacker.
Unfortunately, in the pros, everyone is fast, strong, and has a good football IQ. They don't get fooled, and can easily catch up to a running quarterback. In addition to this, those 230 lb linebackers in college are now closer to 250 lbs, and those 190 lb safeties are 210 lbs or more. That, plus, they all hit hard and fast. Quarterbacks will get annihilated in a short time period running that much in the pros.
One look at a running QB the likes of Cam Newton, tells you a chilling story about how he became an old man in the NFL at the young age of 31, since all of those hits he took running the ball have wrecked his body. This guy was a superb athlete, complete with excellent size and physical strength, and in the end, his reliance on that is what killed off what could have been a two decade career.
I wouldn't doubt it if he were addicted to painkillers at this phase of his life.