Why are D-Series so rare in career mode?

I've been playing career on and off and have only ever found 1 D-Series. Why is that??

 

Probably becuase the population value isn't high. I personally do see the D series about as often as road-train compatible trailers (specifically the short container and short 23ft flatbed, I actually don't know if other vanilla road train trailer configurations exist in-game which I found somewhat strange considering how long middle road train trailers can be in real life) so it's not ultra rare in particular, but I could probably fix that for you as I did here for the pigeon and various other vehicles:
https://www.beamng.com/threads/missing-vanilla-vehicles-in-vanilla-career-mode.96661/