I think becoming a police chief requires a lifetime of dedicated police work. But it also require lots of political skills. Police chiefs do not change that often and when the decision is made to find a new one there are often lots of qualified candidates from both inside and outside the department. Being the best cop does not necessarily mean that you will end up as the next chief.
It's unfortunate that this is the case but that is just the way that power works. It's not like we elect the smartest guy to be President even though that makes a lot of sense.