Why was he producing in Jacksonville but not in Carolina?
Carolina had several issues.
Lets start with the football side of things which I think is more important to understand. The Panthers offense was trying to ask Young to do a lot of things but that OL wasn't stout enough up front. It could be argued that Young compounded the issues of the Panthers interior OL and OL in general. But when you have a QB of that stature you need to be stout in the middle for that type of QB to succeed. The shorter QB's that can succeed behind leaky OLs like Flutie. Wilson or Garcia are super rare. QB's like Young and Brees need protection up front.
They also traded away their top WR in DJ Moore which means the players lower on the depth chart were asked to do more which is not good. Is Thielen a #1 WR at this point of his career? Really he never was a #1 when in his prime. Marshall Jr hasn't shown his potential and Mingo was too fresh to do much. Chark and Shenault are fine as complimentary players but asking them to be more than that isn't realistic.
Also the coaches were split on how to help Young and would go behind Reich's back to the owner, Tepper. Tepper does a lot of meddling which meant there was no unifying direction for the coaching staff going in a million directions.
Canales is going to fortify the center of the OL which they have done and then lean on the running game. Panthers have Hubbard/Sanders which they didn't fully utilize last year and asked Young to do too much.