He lacks maturity and has not played well. And if you listen locally to comments on what the HC said, it was pretty clear that he felt Winston needed some time on the bench to watch and think things over.
As for mobile QBs; I think the proof is self-evident. Very few college QBs who win with their legs ever make it in the NFL.
And more, about the only way they make it is if they are allowed to continue using their legs as a major weapon. The offense needs to be structured around it and the coaching staff needs to be prepared to lose their QB from time to time and likely for him to have a shorter career than typical drop back passers. It's rolling the dice.
Everyone oohs and aaahs at Aaron Rogers wonderful mobility; but look at how many games of being on IR that it has cost him. On the other hand, when he's healthy, he's exciting and he wins. The ironic part is that he is so accurate and good in the pocket, that he needs to stay there and only shuffle around to gain a few tenths to get off a pass. Forget running for gaining yards. Get out of bounds with plenty of room to spare.
Yes, every now and then a mobile guy comes along who is able to avoid regular injury. But if I'm the HC or GM and doing the drafting, the QB with great running skills is not the type of player I'm looking for. Lot's of time and investment in developing a QB; and in building a team around that guy. So when he doesn't pan out, it involves more than just changing QBs.
Pocket passers win Superbowls. Rarely some highly mobile guy comes along that wins a Superbowl because of his legs (Wilson). But I'll plan for the 9 out 10 pocket passers and not the 1 in 10 mobile QB.
So, in the long run I'm not a mobile QB fan.