It is hard to explain all of what makes a manager terrible, but then again it is quite simple.
Some guys just are not up to the job. No matter their history as a great player or great assistant, sometimes the transition just was not or it not for them.
Many a great player has been believed to be destined to be a great manager. However, more often than not, that never works out.
Some magically get job after job despite terrible track records. Others are one and done and luckily the football world has been able to almost forget them—if not for people like me who like to throw it in their face.