Like any organization, the USPS is made up of people.
Some of these people are real saints. Hard working, honest, friendly, and earnest in their duties and eager to serve the customer. Members of the community and proud of it.
Others are lazy, shiftless, or just plain dishonest. Loafing on the union's guaranteed pay for minimal work, looking for excuses to "put it to the man" by finding loopholes to do the barest minimum, grouchy, rude, entitled, and bitter. Petty tyrants with a little bit of power, and overly eager to abuse it.
And it runs the gamut between.
The postmaster and postmaster general for an area will have a strong impact on this; a dishonest, lazy, or inept one will tolerate a lot more dishonesty, laziness, or just plain incompetence from his underlings. An honest, hard-working, in-tune one will run a tight ship. The first will tend to frustrate the good examples into quitting in disgust as they see the bad actors rewarded and they themselves can't catch a break. The second will tend to squeeze out the bad actors, rewarding the hard workers by being aware of their excellence and specifically paving the way for them to do their jobs even better.
The other side of the coin is the union. A union with a boss who is dishonest, sneaky, greedy, or just very skilled at "protecting" the bad actors will make it hard for even a good postmaster to keep his people well-behaved. A union with a boss who genuinely wants to represent the workers will know them and will work as an ally to a good postmaster to make things fair for the best actors, and fair (in the sense that they will get justly and thus poorly treated) to the bad actors as well.
So, it's people. A lot related to the people "at the top" for a region, but it's just...people.