Basically, IIRC, the main issue was his mobility and the fact he's rushed for negative yards. They said every first round quarterback going back so many years has rushed for at least 600 (or whatever the figure was) yards. In a nutshell, they said he doesn't fit the profile for what a first-round QB is these days, and therefore he wasn't a 5-star to them as that represents first-round potnential.
They also said his junior film was not as good as his sophomore film.
You can disagree and I'm sure many of us do, but they did give a pretty detailed explanation with data for why they dropped him.