Jackson never really showed potential to be a great pro. He made impulsive decisions, often very bad ones. In this last game against the Colts, there were clearly several "timing patterns" in which he did not know where the receiver was supposed to be on the route. Commentators described some of his plays as "sandlot football", which seemed appropriate.
Think about the NFL. A lot of folks seem to want to go to the running/passing hybrid quarterback. Daunte Culpepper could have been that, if he had the poise to pull it off. He clearly had the physical skills and the intelligence, but lacked the mental toughness to perform well when down. Even insiders in the Vikes organization confirmed that for me, and saw it firsthand. But overall, I am somewhat suspect of the hybrid idea. It can be wildly successful in college, but the physical talents and skills of college players are not of NFL caliber. When you're trying to pass against NFL linebackers who have the quickness of a college safety, you won't have the same kind of double-threat capability you had back at "Ivy U". You will also have a strong tendency to wind up on the orthopedic surgeon's table, since you now play in a league that encourages its defensive players to cripple key offensive opponents who give them the opportunity.
There will always be some QB's who have the unique combo of pass and run skills. Donovan McNabb and a few others come to mind. But the bottom line is that the QB's main function is to direct the offense and pass. Jackson didn't meet that standard.
