Because there are far more failures than successes.
A lot depends on who drafts him and if he works hard (see Jamarcus Russell).
I'll state right now that I do not think Newton will be the best QB in the NFL 5 years from now. He will have to surpass Brady and Manning (might be retired by then) along with Drew Brees, Matt Ryan, Sam Bradford (pretty darn good rookie season).
If he leaves after this year, he will be bucking a trend: Few QBs who have 1 season of playing experience do well. Matt Cassell is a freak in that regard, but he also was around the NFL for several years (and learning from great coaches) for several years. Not sure Newton has that sort of patience.
I wonder how much his dad will ask for when he gets drafted.