So... what you are saying is that even 'blacks' know that 'white' neighborhoods are better and safer and would rather work there.
Aren't those 'blacks' being racist?
Another route might have nothing but curbside mailboxes so the carrier gets to drive a vehicle and stay out of the weather.
Then there are apartment buildings and office buildings with indoor mail delivery receptacles, air conditioning in summer and heating in winter ~ some carriers prefer those conditions.
My own very black carrier has been on this route for over 20 years. Our biggest ethnic group are Filipinos. There are numerous Koreans and Pakistanis as well as Chinese, Jordanians, and Vietnamese.
He's just about the most senior carrier in Northern Virginia. He's going to retire from this route where everyone respects him. They know this route is difficult ~ all the mail comes from everywhere and everybody has every kind of name on earth. He loves it.