Trying too hard to meet the high expectations put on him and probably a bit of batted ball luck going against him are my 2 best guesses. Most of Low A games are not televised so it’s hard to guess without having watched him lately.
Hence has always been a starter. They just didn’t let him pitch past four innings until last year, and didn’t let him try to stretch out until this year.
As for Wetherholt, it may be the same injury that caused him to miss parts of the last two college seasons, as the Cardinals lauded him for playing through injury. Maybe now that his offense has cratered, they’ve decided to let him rest and recover.