What's going on is that the old system of using walking carriers with relay boxes simply did not meet the needs of modern mail volumes. Used to be a route might have 500 stops with each one getting 3 or 4 pieces. Then we started having routes where those 500 stops were getting several times that volume in mail.
You could build relay boxes every block or two and send guys in trucks out to fill them, or you could simply give the carriers a vehicle big enough to hold all the mail at the start of the route, and let them drive.
It's strictly economics with a bow to carrier safety. These guys are on the streets and there are crazy drivers out there.