Well, one of the reason of this problem, is that when you have a bad manager, there are two ways of getting the manager to the door, that is up, or down.

Making him go down is quite hard. You have to build your case against him/her, and it will take quite some time. Plus, if you do such good job at your work that he can take the credit from, this may not be easy. Then again, if the situation is not okay in your job (because of him), it won't be any easy either. That is not counting his/her personal/political skills yet.

On the other hand, making him going up the ladder is much easier. Assuming that you are good at your job, so much that he can take credit from and then doing good reviews of him when you can, there is a good chance that he'll be promoted. While he will leave a train wreck somewhere else, he would leave you in peace, at least for now.

For that reason alone, I guess that is the reason why bad managers can go to the top, at least where incompetence would not have much effect on the bottom line. (ie. monopoly, having good people on the operational line, etc)