1. He was called up well before he was ready (out of necessity). He did an admirable job considering, but being thrown into the fire before he was ready messed with his development.
2. In 2016, he was sent down after 6 freaking games. SIX! If that's not bad enough, the Sox then decided he was going to become a left fielder.
3. After healing from his injury, the Sox said they were committed to playing him only as a catcher. How long did that last? Next thing you know, he's going to be tried out at 1B, 3B, 2B, etc.
Now maybe he wasn't progressing as quickly as the Sox would have liked him to. I really don't know. But either way, he was not given a fair chance as a catcher.