Because Winston can command a huddle, call protections, audible, take snaps from under center, read a defense, manipulate safeties with his eyes. (Though he is reckless going over the middle on the intermediate balls sometimes). He's extremely pro ready and just needs to tighten up his mechanics, which he's never done due to also playing baseball. He's extremely football smart, knows the ins and outs of every play and where every player should be. Will devour a pro playbook and live in it, and understand it, despite the hoopla about his off-field crap.
Bryce Petty has not had a legit playbook in college. He has no idea what he's looking at when he lines up, has never run a real offense or even taken snaps from under center really, only has to read half the field on any given play and "reading" is really a generous term when you consider the mismatches the Baylor WR's create for him and the simplicity of the offense. He has all the same football intelligence questions that Griffin had/has except he's nowhere near the athletic or throwing talent (though he does throw a nice ball in a vacuum).