Each country has their own circumstances to deal with. This design would not make sense in Europe where a lot of the last mile delivery was done by mail carriers on foot or bicycles. However it does boost efficiency considerably in the USA's vast expanse of suburbia. FWIW, mail trucks in Canada also have with right hand drive and low floor so the driver could reach into mailboxes without having to leave their seat.

My local postal service used to make delivery on bicycles but they have gradually introduced golf-cart like miniature EVs. This allow each run to cover a much larger area but probably would not for the most of US because their mail routes are too long.