I agree that he was an option and I am sure the front office thought about it long and hard.
The argument to not use him is twofold:
1. His development is prioritized by letting him continue to start in the AFL. He will get more innings this way. He can start as part of a regular routine and work on things. He really did have a nearly lost season with multiple nagging injuries...
2. The MLB bullpen is deep so the need for him is not GLARING.
Of course with his MiLB K rate and FIP you can certainly argue that he would be a massive upgrade on someone in the pen. There is merit to that. You can also argue that he would get a unique kind of experience/development by being in the MLB bullpen. But on the performance side, with his fringe command right now there is a chance he is actually not a shutdown MLB reliever at this moment in time.