Why fed department heads not promoted from within?

I hope this is the correct sub-forum for my question.

Department directors (CIA, etc) are sometimes appointed from outside of the department.

While I realize that these 'outside directors' often do a great job, I'm a bit surprised that the top position is not filled from within the department more often.

What are the reasons for appointed an outsider?

Thanks for any perspective on this.

 

If you want a good example of the problems with career bureaucrats look at the British comedy "Yes Prime minister"

But I agree the American system of overturning all the appointments seems weird

 

Thank you for the reply.

 

When presidents do that, they are doing it for political reasons. They put in political allies because they want loyalty and political cover more than they want experience and competence. This lends itself to poor choices and poor outcomes. Hillary Clinton as Secretary of State was a good example of that.

 

That makes sense. Thank you for the reply.