The Peter Principle states that a manager will get promoted to his level of incompetency, then get stuck there. As time goes on, more and more managers reach their level of incompetency, until the entire management level is populated by people who are bad at their jobs.

Say a guy is a great salesman, the best in the company, beats all his targets. He’s just great at it, and all his customers love him. So he gets promoted to sales manager, with a nice office, and a big raise.

But just because he’s a talented salesman, doesn’t mean he knows how to manage sales people. Also, the guy who replaced him isn’t pulling in near the revenue, so the department he’s managing is now WORSE than before he took over. He becomes just another mediocre manager, in over his head, and he never gets another promotion again.

Or maybe he moves up a few more levels, but eventually he hits the point that he can’t do well, and that’s where he remains, being mediocre. After a while, ALL the managers are people who were good at their old jobs, but mediocre at this one, and here they stay, until they are swept out in their 50s.