I think a lot of it has to do with density. In the urban areas of the US mail carriers walk, lot sizes are small (8-10m) so getting on an off a bike every 30 seconds doesn't make much sense. In the rural areas it could me a mile or more between houses. They both can use the same truck, in urban areas they just park it in a central location and refill their bag as needed then drive it back to the shop when done.