The Peter Principle: People rise to their level of incompetence.

Let's say you're a great salesman, the best in the company, so they make you a Sales Manager. Unfortunately, that's not your strength, and you do a mediocre job, so you don't get promoted beyond that position, where you stay for years, being mediocre.

Eventually, all the middle manager positions, and maybe even the upper management positions get filled with people who were great in past jobs, but only mediocre in this one, and the entire company settles into mediocre business management.

That's the Peter Principle.