I think there's two parts to Mac Jones' downfall here.
1) I think he was very physically limited. I don't think he ever had a high ceiling. I think his starting point in terms of mentally understanding the game was pretty far ahead of where most rookies are. But his physical limitations gave defenses an advantage because his lack of mobility or ability to make tough throws allowed them to throw more at him and just physically dominate him.
2) The personnel around him and coaching staff they hired to develop him were subpar. The coaching staff literally didn't have experience on offense in that second year. I really can't put into words how asinine, ridiculous, preposterous, etc. that was. I'm as big a fan of BB as there probably is on this board, I don't think I've ever seen a more obviously and thoroughly terrible decision by a coach than that. It's one thing for a decision to not work out or be misguided but usually there's some logic in it. There was no logic in that. Even with a resume as great as BB's is, it wouldn't have been out of line to fire him solely for that decision because it might be the dumbest coaching move in recent memory. It legitimately pointed to incompetence. And beyond that, the personnel wasn't strong enough to make up for a QB that had limitations. Not even close.
I think Mac was able to see as clearly as we were how dumb the coaching decisions were and the relationship never recovered from that point. Honestly, it might have been for the better because if they had hired an actual offensive coordinator and did a decent job of adding talent around him, I fear he might have ended up Daniel Jones where the team and player were juuuuuuuuuuuuust good enough where you keep the relationship going until it eventually fizzles out like that situaton will.