If you look at the percentile rankings on Statcast, there's a lot of red. The only knock against him has been his bb rate. If he could reign in the walk rate, he could be a really great asset. Unfortunately, if you look at his annual stats, they are always less than desirable because of all those walks. It's the same reason that they sent DHern down. If you're walking the park, it doesn't matter how many k's you get.
https://baseballsavant.mlb.com/savant-player/jake-diekman-518617?stats=statcast-r-pitching-mlb