Edit: Yes, yes, a ton of players have plenty of potential, but never achieve it. That's the harsh reality of football, or anything in life, for that matter. Still, it's worth considering that there were several unfortunate events happened that derailed his ability to be a good QB.
1) Not playing his senior year due to the COVID-19 related cancellations in California. Most good prospects take a significant, much-needed leap in experience and ability during that time, and his inactivity stunted his growth as a QB.
2) Tommy Rees being his QB coach and trying to "fix" his mechanics, was probably the biggest factor here.
Looking at his film from his junior year in high school, he'd sling the ball downfield with great zip and pretty good accuracy. After his throwing mechanics were changed by Rees, it looked as if every throw were a long windup struggle, that his throwing velocity wasn't quite as good, and his accuracy became suspect. Combine this with his inability to read defenses properly (even after spending most of the season in the OC booth with Rees), and what we saw in the heart attack-inducing Gator Bowl would have been his norm.
If anything, this degradation looked like a painful reminder of how Brandon Wimbush's accuracy got worse over time. A good QB coach is supposed to be able to at least halt the degradation, and help begin some improvements.
3) The shoulder injury, and missing most of his sophomore year, robbed him of an opportunity to really take command.
4) Following Tommy Rees to Alabama, where again, Rees' limitations as a QB coach all but sealed his fate. The QB situation at Alabama wasn't due to the lack of talent. Jalen Milroe is a very good downfield passer, and Ty Simpson may very well be the best of the bunch, but Tommy Rees kept trying to turn everyone into a dink 'n dunk, nibbling passer in his own image. It wasn't until Rees finally "allowed" Milroe to throw downfield (I suspect this had more to do with Nick Saban issuing that order) that Milroe flourished.