Great article, Dianne! I think another reason managers fail to manage is that they don't know how. This seems especially true when one is promoted to manager because one did a good job as an employee. Companies often fail to train managers to manage, and expect them to do a good job managing the tasks they performed well without any regard for the art & science of managing.
I think it's sad that many managers are set up to fail because of this, too, when just a little leadership and management training could really work wonders. Fortunately, there is a lot of info out there that managers can find and self-study if they really want to succeed. (That plus a healthy dose of trial & error and a lot of time invested is about how I learned!)