Not a Mistake: Big Cass was overrated.
Big Cass is certainly big. It's right in the name. But given the fact that "Big" is all that WWE could come up with to say about Morrisey speaks volumes.
While large and powerful, Cass fails to move with the agility and elegance of other big men such as Braun Strowman or Luke Harper. He often relies on his opponent, such as Daniel Bryan, to make his matches convincing. In short, he wasn't a big draw and wasn't likely to improve his in ring technique, so firing him cost the WWE nothing.