I think it has more to do with the times than anything. WORK trucks were WORK trucks... No frills. People weren't so afraid back then of having their truck stolen. Most probably never locked their truck anyway.
My 48 has a three position ignition switch... Locked, On, and Unlocked. You could take the key out in any position. If you take it out in the On position, you can still shut it off and turn it back on... so you don't even need a key.
Working men probably didn't even want to bother with fumblin' around for a key... Let alone locking a door.
