The most logical theory as to why the passenger door was the only standard-equipment door with a lock:
- In pre-Interstate highway years, most trucks drove close to home; and, close-to-home roads were two-lanes.
- It was common/safer to get out and in on the passenger side (rather than get in/out in the road).
- So, you lock the driver's door from the inside.
- Slide over the seat to the passenger side, and get out on the passenger side.
- Then, lock the passenger door from the outside.
