Two things are holding him back in my opinion.
His atrocious BB/9 rating. I don't know if it is warranted or not but that is extremely low. He is the highest rated reliever with a BB/9 attribute that low.
The only live series diamond relievers in the game are all flagged as closers. As long as the brewers have Hader then Williams will have trouble reaching diamond. This is not really fair but that is how it seems.