re: The question is, why did it take Brady to change the offense?

Miles was stubborn and simply refused to change.

O tried to change things immediately when he hired Canada. That obviously didn't work out. O then hired a guy he was comfortable with (Ensminger) to run the offense in order to avoid a repeat of 2017 with the issues between O and Canada.

However, Steve, who had a significant career previously as an OC was kind of set in what he liked to run. Once it was clear that offense wasn't going to be good enough O (and Steve) when looking for help to "modernize" the approach. They found it in Brady who then stepped into a PERFECT situation to fully implement and execute his approach. Experienced QB; Very talented WR's who now had experience; An experienced OL and an underrated jack-of-all trades RB to give LSU a major weapon.