Why the /6

Because I like it!
Need more than that?
Good fuel economy, great low end torque, cheep, simple, different, original, and to simple to quit even when broken!

"not a great engine for a pickup if you load it up and want to go up hills.." (dartnut)
The one I had in my 3/4 ton crew cab pu didn't have any trouble pulling hills at 65-70 with ~6k in the bed and ~7k in a 16' tandem axle trailer in back. Went over 600 miles loaded that way.