This.
This idea that growth is linear? Not true.
This idea that there is always a person to blame? Not true.
Stuff happens. Random stuff. Injuries. Coaching changes. Adjustments. Etc......
The most likely, simplest, explanation is that a WHOLE LOT of bad things have happened, injuries, adjustments (by him and pitchers and coaches, and him to play thru injury), inconsistent playing time, etc.....all these things add up. And even then, the simplest explanation is that MLB is HARD, and sometimes things don't work out right away or at all.