I have a theory about that. I actually come from a long line of postal employees although I am not one of them.
For one thing, it's damn near impossible to get fired. You can be the most whacked out person alive, and they can't fire you unless they give you x number of warnings first and do all of the necessary paperwork. Nobody wants to deal with the BS so everything just stays the same with 'asshole' still employed and everyone else either hating him or scared to death of him.
Everything there is very unionized, civil service oriented. There's typically not that many employees at any one location. The only way you ever get a promotion is if some leaves or dies. They your boss goes up one notch, you take his job, the guy beneath you takes your job etc.
All it takes is one irregularity in this slow, plodding carreer path and then SNAP! We're watching them haul out the bodies on the evening news.
Just my theory, but I'm sticking to it.