Maybe because Newton never really progressed beyond being a bulldozing running threat with a live arm that could zip it to beat zone coverages. Never really manipulated defenses or threw with anticipation or improved himself from the pocket once teams figured out how to minimize what he did best.
Milton looks to me like Allen's first year in what hes asked to do and what he's executing. I don't think he's the threat Newton (or Allen) is with the legs, but if he learns the position better than Newton did, for sure he can be a plus starter. He's got a high ceiling.